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

[EX Q]: Passage: A front forms at the boundary between two air masses. Types of fronts include cold, warm, occluded, and stationary fronts. Clouds, precipitation, and storms commonly occur along fronts. Answer: a front
[EX A]: What forms at the boundary between two air masses?

[EX Q]: Passage: Equinox comes halfway between the two solstices. At equinoxes, the Sun's rays shine most directly at the Equator ( Figure below ). The daylight and nighttime hours are exactly equal on an equinox. The autumnal, or fall, equinox happens on September 22 or 23. The vernal, or spring, equinox happens March 21 or 22 in the Northern Hemisphere. Answer: equinox
[EX A]: What event occurs between the two solstices?

[EX Q]: Passage: Plants have evolved three pathways for carbon fixation. Answer: three
[EX A]:
How many pathways do plants have for carbon fixation?