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

Ex Input:
Passage: Dissolved mineral salts wash into the ocean. As ocean water evaporates, it leaves the salts behind. This makes the water saltier. Ocean water is about 3.5 percent salts. The main salt is sodium chloride. Answer: salts

Ex Output:
Concentration of what, the substance left behind when ocean water evaporates, is about 3.5 percent?

Ex Input:
Passage: Heat waves have increased in frequency and duration in recent years. The summer 2011 North American heat wave brought record temperatures across the Midwestern and Eastern United States. Many states and localities broke records for temperature and for the most days above 100 o F. Answer: cell membrane

Ex Output:
What are all cells surrounded by?

Ex Input:
Passage: Drip irrigation uses much less water than other methods. Answer: drip

Ex Output:
Which kind of irrigation uses much less water than other methods?