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

Example Input: Passage: The male gametophyte produces flagellated sperm that must swim to the egg formed by the female gametophyte. For this reason, sexual reproduction must happen in the presence of water. Therefore, nonvascular plants tend to live in moist environments. Though the life of a nonvascular seedless plant is a cycle, this can be considered the initial step in the life cycle. Answer: water
Example Output: When flagellated sperm must swim to the egg, sexual reproduction requires the presence of what substance?

Example Input: Passage: Figure 2.43 Positions and velocities of a metal ball released from rest when air resistance is negligible. Velocity is seen to increase linearly with time while displacement increases with time squared. Acceleration is a constant and is equal to gravitational acceleration. Answer: acceleration
Example Output: If a ball is released from rest when air resistance is negligible, velocity is seen to increase linearly, while what related property is a constant?

Example Input: Passage: Seismic waves show that the outer core is liquid. The inner core is solid. Answer: it is solid
Example Output:
What do seismic waves prove about the inner core?