求解新GRE《阅读逻辑10套》EX4第五题的D选项!
People who engage in scuba diving are healthier,on average,than people who do not engage in this activity.Therefore,scuba diving tends to promote improved health.
The argument is most vulnerable to criticism on the grounds that it
（A）presupposes that everyone who takes up scuba diving does so solely for health reasons
（B）leads to a further and falsifiable conclusion that no one can achieve good health without engaging in scuba diving
（C）fails to point out that a small number of people are seriously injured in scuba diving accidents each year
（D）treats a precondition for improving one‘s health as though it were something that by itself could ensure good health
（E）overlooks the possibility that people generally do not take up scuba diving unless they are in good health
为什么不选D?选项里的precondition是指什么啊?