Read the passage below. Earth and Mars are known as terrestrial planets because they are both made up of rock and metal. However, scientists know much more about the internal structure of Earth than they know about the internal structure of Mars.

What is the effect of the comparison signal word "both”?

It signals that Earth and Mars have something in common.
It signals that Earth and Mars are different from one another.
It signals that scientists studied Earth before studying Mars.
It signals that more facts about Earth and Mars are coming.