At night, the solar energy (stored as vast quantities of heat in city buildings and roads) is slowly released into the city air. Additional city heat is given off at night (and during the day) by vehicles and factories, as well as by industrial and domestic heating and cooling units. The release of heat energy is retarded by the tall vertical city walls that do not allow infrared radiation to escape as readily as does the relatively level surface of the surrounding countryside. The slow release of heat tends to keep nighttime city temperatures higher than those of the faster-cooling rural areas.

The word "retarded" in the passage is closest in meaning to

Several dinosaur fossil localities preserve coprolites. Coprolites yield unequivocal evidence about the dietary habits of dinosaurs. Many parts of plants and animals are extremely resistant to the digestive systems of animals and pass completely through the body with little or no alteration. Study of coprolites has indicated that the diets of some herbivorous dinosaurs were relatively diverse, while other dinosaurs appear to have been specialists, feeding on particular types of plants. The problem with inferring diets from coprolites is the difficulty in accurately associating a particular coprolite with a specific dinosaur.

The word "unequivocal" in the passage is closest in meaning to

Receptors for the sense of smell are located at the top of the nasal cavity.

Passenger ships and aircraft are often equipped with ship-to-shore or air-to-land radio telephones

Dotting the marshy expanse of the Florida Everglades are little islands known locally as hummocks.

It is not possible for people to recall everything that they have thought, felt, or done.

When preparing a diet, a person should be aware that vitamin D acts to increase the amount of calcium absorbed by the body.

A new atomic clock being developed for navigation satellites will perform better than previous devices. The clock will use a new microwave cavity design to provide a compact and lightweight package and new electronic techniques to maintain long-term stability. The clock can provide precise navigation information because it is stable to one second in three million years. The differences in the time when signals from four satellites arrive at one location can be used to calculate that position to within a few yards

From the passage, it can be inferred that which of the following characteristics of the clock mentioned will be most impressive?

A new atomic clock being developed for navigation satellites will perform better than previous devices. The clock will use a new microwave cavity design to provide a compact and lightweight package and new electronic techniques to maintain long-term stability. The clock can provide precise navigation information because it is stable to one second in three million years. The differences in the time when signals from four satellites arrive at one location can be used to calculate that position to within a few yards.

It can be inferred from the passage that the new clock will be

A new atomic clock being developed for navigation satellites will perform better than previous devices. The clock will use a new microwave cavity design to provide a compact and lightweight package and new electronic techniques to maintain long-term stability. The clock can provide precise navigation information because it is stable to one second in three million years. The differences in the time when signals from four satellites arrive at one location can be used to calculate that position to within a few yards.

According to the passage, signals from how many satellites will be used to calculate a position?

What is the primary purpose of the passage?