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

Ex Input:
Passage: boundary between air masses that is stalled in one place. Answer: aids

Ex Output:
Opportunistic diseases are usually the direct cause of death of people with what disease?

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: Brain injuries can range from mild to extremely severe, but even mild injuries need medical attention. Brain injuries can result from falls, car accidents, violence, sports injuries, and war and combat. Falls are the most common cause of brain injuries, particularly in older adults and young children. Answer: falls

Ex Output:
What are the most common causes of brain injuries?