ADMICRO

Read the passage below carefully, and then choose the best answer:
The Most Powerful Force in Space
A black hole is a spot in space that has incredibly powerful gravity. A hole’s gravity is so strong that it pulls everything nearby into it. It can pull stars, planets, and other things. Nothing can escape a black hole. In fact, they are so powerful that even light cannot escape them. Scientists first started talking about black holes in the year 1783. That year, one scientist said that in the universe, there might be places with gravity strong enough to trap light. However, that scientist didn’t use the term “black hole”. The term “black hole” was invented in 1967. Black holes are formed when a star dies. When a star dies, it collapses. When that happens, a huge amount of matter crowds into a very small space. The matter becomes very, very dense. How much matter are we talking about? Imagine a star that is the size of ten suns. Imagine fitting that amount of matter into New York City. That is what a black hole is like. Its gravity is so powerful because it has so much matter. However, not all black holes are huge. Scientists believe that some black holes are the size of atoms. Although these black holes are tiny, they might have the same amount of matter that a mountain has. We can’t see black holes because they don’t have any light. They are completely invisible. Scientists can guess where black holes are by studying nearby stars and planets. If gravity seems to be pulling the stars and planets to one spot, then scientists believe that the spot is a black hole. Some people wonder if a black hole could ever destroy the Earth. We do not need to worry about that. There is no black hole that is close enough to destroy the Earth. The closest black hole is about 7,800 light years away. In addition, the sun is not big enough to create a black hole.
5. The word “them” in paragraph 1 refers to _________.

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