We can't see black holes because D) no light can get out
Explanation:
Black holes are the result of the gravitational collapse of a supermassive star.
The life of a supermassive star ends with a huge explosion, called supernova, that leaves behing a super-dense core called black hole.
Black holes are the most dense objects of the universe, having a huge mass in a super small size. For this reason, the gravitational force exerted by a black hole in its proximity is so strong that even light is not able to escape from the gravitational field. For this reason, light from a black hole is not able to reach us, and so we are not able to see these objects.
The "edge" of space beyond which light remains "trapped" inside the black hole is called event horizon, and no object can escape this region of space.
The radius of the event horizon of a black hole is called Schwarzschild radius and it is given by:
where
G is the gravitational constant
M is the mass of the black hole
c is the speed of light
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