Pls hurry, i'm timed TmT

Read this excerpt from "Why Are They Called Black Holes?"

Black holes are invisible because they have so much gravity that nothing escapes. Not even light can escape or bounce off. So how do scientists know that black holes exist if they are invisible?

Gravity is caused by matter, and as matter increases, the pull of gravity increases too. Stars are born when enough matter collects that gravity makes it collapse into a hot, dense protostar.

According to the introduction, why are black holes invisible?

Scientists can not figure out why they exist.
Gravity is caused by matter.
They have so much gravity that nothing escapes.
Stars are created first.