Given the task definition, example input & output, solve the new input case.
Given a scientific passage and an answer, generate a question for the given answer.
Example: Passage: A frameshift mutation is a deletion or insertion of one or more nucleotides that changes the reading frame of the base sequence. Deletions remove nucleotides, and insertions add nucleotides. Consider the following sequence of bases in RNA:. Answer: nucleotides.
Output: A frameshift mutation is a deletion or insertion of one or more of what that changes the reading frame of the base sequence?
The science passage defines frame mutation and the main compoment here is nucleotides. Hence the generated question is apt.

New input case for you: Passage: Currents also flow deep below the surface of the ocean. Deep currents are caused by differences in density at the top and bottom. Density is defined as the amount of mass per unit of volume. More dense water takes up less space than less dense water. It has the same mass but less volume. Water that is more dense sinks. Less dense water rises. What can make water more dense?. Answer: density
Output:
Deep ocean currents are caused by differences in what property at the top and bottom?