Health is something we tend to ignore when we have it...

Câu hỏi: Read the following passage and mark the letter A,B,C or D on your answer sheet to indicate the correct word or phrase that best fits each of the numbered blanks from 1 to 5.
Health is something we tend to ignore when we have it. When our body is doing well, we are hardly aware of it. But illness can come even when we are young. In fact, childhood has been a very susceptible time. (1) _________ diseases attack children in particular, and people know very little how to cure them once they struck. The result was that many children died. About a century ago, (2) __________, scientists found out about germs, and then everything changed. The (3) ___________ of many diseases was found, and cures (4) _________ were developed. As this medical discovery spreads, the world became much safer for children. The result is that whereas hundred years ago, the average man lived for 35 years, nowadays, in many areas of the world, people can expect to live for 75 years. And what do we expect by the year 2020? Undoubtedly, medical science will continue to (5) _________ Some people will be able to avoid medical problems that are unavoidable today.
The word or phrase which best fits the gap (3) is...
A. source
B. origin
C. reason
D. cause