What does the passage mainly discuss?

• The similarities between Earth's inner core and outer core

• The structure and temperature of Earth's interior

• When seismic waves were first used to study Earth's interior

• Why Earth's solid inner core is surrounded by a molten outer core

The word "mere" in line 2 is closest in meaning to

• approximate

• insignificant

• measured

• lengthy

According to the first paragraph, most knowledge of Earth's interior has been gained by studying

• Earth's gravitational field

• Earth's magnetic field

• vibrations caused by earthquakes

• material taken from holes drilled into Earth's crust

According to the second paragraph, which of the following statements regarding Earth's inner core and outer core is true?

• Neither the inner core nor the outer core can be studied using seismic waves

• The outer core is more solid than the inner core

• The inner core and the outer core have greatly different temperatures

• Both the inner core and the outer core probably contain iron

The word "thoroughly" in line 15 is closest in meaning to

• basically

• similarly

• potentially

• completely

The word "it" in line 18 refers to

• Earth

• a cool crust

• the molten ball of rock

• the heat

The word "functioned" in line 18 is closest in meaning to

• acted

• moved

• appeared

• grew

The word "manufactured" in line 20 is closest in meaning to

• changed

• combined

• utilized

• made

The third paragraph mentions which of the following as one cause of the current of the current high temperature of Earth's interior?

• The decay of radioactive isotopes

• The movement of elements from Earth's crust to its interior

• The vibrations that result from movement of Earth's crust

• The injection of certain organic elements into Earth's inner core

Look at the terms "seismic waves" (line 6), "earthquakes" (line 6), "magnetic field" (line 14), and "stable elements" (line 21). Which of these terms is defined in the passage?

• seismic waves

• earthquakes

• magnetic field

• stable elements