instruction:
In this task, you need to write a topic word from the given fact. The topic word must have at least one word overlap with the given fact. The topic word often involves adding a new word from a related concept. In your topic word, use at least one word from the given fact. Topic words with two or more words work best.
question:
Fact: propulsion is used for flying by a spacecraft.
answer:
propulsion spacecraft.


question:
Fact: sexual reproduction increases genetic diversity.
answer:
genetic diversity creates evolution.


question:
Fact: gases from burning coal and oil dissolving in water in the atmosphere causes acid rain.
answer:

Acid is defined as.