What is a scientific question?
a question that is asked by scientists only
a question that is based on observations and that is testable
a question that is broad and that reinforces existing knowledge
a question that has a simple answer that doesn’t need to be tested

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

the correct answer is - d. china has a large and growing population.

the fact that china has a large (the biggest) population, and it is still growing, doesn't hinder the agriculture production in the country. in fact, it can for increased production in the agriculture, since there's more labor force, as well the sheer mass of people produces lots of scientists that come up with new inventions as to how to improve this sector. nice example is the inner mongolia region in china, in general dominated by the gobi desert, but few scientist came up with a nice idea as to how to make the region productive by using plants that are very resistant to droughts, so lots of people have been employed, and the region is becoming greener and greener each year, and the production of food in this region is also increasing.