Given a scientific passage and an answer, generate a question for the given answer.

[EX Q]: Passage: 19.3 The Human Population Earth's human population is growing exponentially. Humans have increased their carrying capacity through technology, urbanization, and harnessing the energy of fossil fuels. The age structure of a population allows us to predict population growth. Unchecked human population growth could have dire long-term effects on human welfare and Earth's ecosystems. Answer: carrying capacity
[EX A]: Human population is growing exponentially and humans have increased what normally limiting factor through technology, urbanization, and harnessing the energy of fossil fuels?

[EX Q]: Passage: An autoimmune disease is a disease in which the immune system attacks the body's own cells. Why this happens is not known for certain, but a combination of genetic and environmental factors are likely to be responsible. Type 1 diabetes is an example of an autoimmune disease. In this case, the immune system attacks cells of the pancreas. Two other examples are multiple sclerosis and rheumatoid arthritis. Answer: autoimmune
[EX A]: What term describes a disease in which the immune system attacks the body's own cells?

[EX Q]: Passage: All machines make work easier, but they don't increase the amount of work that is done. You can never get more work out of a machine than you put into it. In fact, a machine always does less work on an object than the user does on the machine. That's because a machine must use some of the work put into it to overcome friction. Friction is the force that opposes motion between any surfaces that are touching. All machines involve motion, so they all have friction. How much work is needed to overcome friction in a machine depends on the machine's efficiency. Answer: friction
[EX A]:
What is the term for force that opposes motion between any touching surfaces?