(a) Black holes are invisible. Why? How are they identified?
(b) How do multistage rockets differ from single-stage rockets? Explain

Hint: a) Black holes are those which can absorb all the radiations into it and can be detected with the help of an X-ray telescope.
b) A multistage rocket is a rocket that uses two or more phases, each containing its own engines and propellant

Complete step by step answer:
(a) Black hole is invisible, this is so because of the strong gravity that can even pull the light also into itself, and without light, it can never be visible.
Moreover, if a black hole passes through a cloud of matter between stars, it will drag the matter in through a process known as accretion. In this case, the black hole can tear the star as you pull it into it. As this is quick and hot, it emits x-rays that leave space that can be detected by X-ray telescopes. Dark holes have a profound effect on their surroundings. They emit powerful gamma-ray explosives that can also be detected.

(b) A multistage rocket is a rocket that uses two or more phases, each containing its own engines and propellant. The main reason for the multi-stage rockets and boilers is that when the fuel is burned, space and structure it contains and the motors themselves are useless and only add weight to the car which slows down its future speed. By discarding useless sections, the rocket illuminates itself. Focusing on future phases can provide even greater speed if the early stages were still attached, or more than one large rocket. Upon descending the stage, the remaining rocket is still moving near the speed the entire assembly reached at the time of the fire. This means that it needs a little more fuel to reach the given velocity and/or height. In contrast, a single-rocket rocket uses a single rocket motor or a set of motors to lift its load to the desired height, and most of the loudest rocket rifles are single-stage.

Note:
Remember that the density of a black hole is infinite and its gravity is immense. That's why light can’t escape its gravity.
Fact: Astronomers found stars can form in stuffed blast out from near supermassive black holes.