Determine the Main Idea 1

Blooms in Death Valley

In March 2016, Death Valley National Park in California experienced a rare occurrence: thousands of yellow wildflowers covered the region in a "super bloom." Why was this unusual? Death Valley is the hottest and driest place in North America. The area's average yearly precipitation is just two inches, making it a challenging place for flowers to grow. However, in October 2015, the valley had received more than three inches of rain in some areas. As a result of the unusually wet season, wildflower seeds that had been lying dormant underground suddenly began to grow, producing an abundance of golden blooms.

What is the main idea of the passage?

• Because of an unusual amount of rain, thousands of wildflowers grew in Death Valley in 2016.

• Because of the hot, dry conditions in Death Valley, it is difficult for flowers to grow there.

Tags

CCSS.RI.7.2

AC vs. DC

In the 1880s, the War of the Currents began. Thomas Edison developed direct current, or DC, a type of electricity that flows in one direction. By 1887, DC was in use around the United States. It had some drawbacks, though. Converting it to lower or higher voltages was difficult; that is, the current's force couldn't be changed easily. Transmitted at the low voltage needed for use in homes and businesses, it lost power after traveling only a mile. Meanwhile, Nikola Tesla was working with alternating current, or AC. AC reverses direction many times per second and can be changed to different voltages. At high voltage, it can be transmitted efficiently, traveling hundreds of miles with little energy loss. Because of its advantages, AC triumphed by the end of the nineteenth century.

What is the main idea of the passage?

• AC has distinct advantages over DC, so AC won the War of the Currents.

• AC reverses direction and can be converted to lower or higher voltage.

Tags

CCSS.RI.7.2

African Wild Dogs: Ultimate Predators?

Many books, papers, and television shows have created an image of the African wild dog as the ultimate predator. According to these sources, the dogs' coordinated teamwork and their unusual endurance help them catch about eighty percent of their prey. A 2012 study tested that notion. Researchers fit a pack of wild dogs with special collars that recorded the dogs' positions and speeds. The data that the collars recorded allowed the researchers to learn what the dogs did when they weren't being observed. The results provided a surprising contrast with the dogs' fierce reputation. The dogs never used teamwork in their hunts; they chased their prey over short distances as opposed to long runs; and instead of eighty percent, they brought down only about sixteen percent of their prey.

What is the main idea of the passage?

• A study shows that African wild dogs catch about sixteen percent of their prey and do not use teamwork in hunting.

• African wild dogs have a reputation as fierce hunters who work in teams, but new data suggests otherwise.

Tags

CCSS.RI.7.2

CCSS.RI.7.1

he Evolving Human Skeleton

Our early ancestors were hunter-gatherers who obtained food by hunting and by searching for edible wild plants. They needed strong bones, including large, strong jaws that enabled them to eat tough, uncooked foods. When our ancestors developed agriculture, however, their diet changed. They began growing plants, such as grains and beans, and raising animals for food; they also started cooking their foods, making them softer and easier to chew. Consequently, the human skeleton underwent radical changes. Over time, the human jaw became smaller and changed shape. Other bones also evolved, becoming lighter, especially in the joints, as result of both dietary changes and a less active lifestyle.

What is the main idea of the passage?

• Early humans ate uncooked meats and vegetables, so they needed strong jaws for chewing.

• As early humans developed agriculture, their bones changed significantly.

Tags

CCSS.RI.7.2

Making Towers Bird-Friendly

The lights on tall communication towers warn pilots to avoid the towers when flying at night. Unfortunately, the steady red lights often used on towers have the opposite effect on birds: the bright beams attract and confuse them. Instead of avoiding the lights, the birds fly directly toward them, crashing into the towers or becoming entangled in power lines. Millions of birds have died this way. However, there is a solution. Unlike steady lights, flashing or blinking lights don't attract birds. In response to demands from conservation groups, the Federal Aviation Administration in 2016 began requiring communication towers in the U.S. to use blinking lights instead of steady ones.

What is the main idea of the passage?

• Birds are attracted to the steady red lights on communication towers.

• To save birds, communication towers have begun using blinking lights.

Tags

CCSS.RI.7.2

CCSS.RL.7.2