4.
Which line in the passage uses a synonym as a context clue to explain the meaning of the word exploration?

Artificial Satellites

A satellite is an object in space that rotates around a bigger object. For example, the moon is Earth’s satellite. The moon, however, is a natural satellite. The ones we send from Earth into space are called artificial satellites. Sputnik, sent by the Soviet Union, was the first artificial satellite to go into space. Since then, we have sent several satellites into space for various exploration purposes. Some satellites help by investigating faraway objects that may be heading toward Earth. Others help to study the weather conditions of Earth. They also observe the sun and other stars and planets in space.