Why can black holes not be seen?

they have too much gravity for light to escape.
they are hidden in interstellar dust.
they exist only behind other stars.
scientists don’t know where to look for them.

The starting materials in a nuclear change have a total mass of 2.3465 x 10-27 kg. after the change, the resulting materials have a mass of 2.3148 x 10-27 kg. how much energy was released during the change? 2.85 x 10-12 2.08 * 10-10 j 0 211 * 10-10 j 8.56 * 10-43