Read the following passage and choose the most suitable option for each of the blanks.
Health is something we tend to (1) _____ when we have it. When our body is doing well, we are hardly (2) _____ of it. But illness can come, even (3) _____ we are young. In fact, childhood has been a very susceptible time. Many diseases attack children in particular, and people know very little (4) _____ to cure them once they struck. The result was that many children died. About a century ago, (5) _____, scientists found out about germs, and then everything changed. The (6) _____ of many diseases was found, and cures were developed. As this medical discovery spread, the world became (7) _____ safer for children. The result is that (8) _____ a hundred years ago, the average man lived for 35 years, nowadays, in many areas of the world, people can (9) _____ to live for 75 years. And what do we expect by the year 2020? Undoubtedly, medical science will continue to (10) _____. Some people will be able to avoid medical problems that are unavoidable today.
Trả lời câu hỏi dưới đây:
______(4)
A. cái mà
B. cái/ vấn đề
C. như thế nào
D. khi