#17: the question stem is a NOT weaken. So we are looking for the answer choice that does not weaken. I am going to skip over (C) because you state that you know why that answer choice is correct. Answer choice (D) weakens (has a result of decreasing the risk) because it is weakening the aspect of having made corrections for age and smoking habits. While the stimulus states that the study was corrected for age and smoking habits, choice (D) weakens by stating that heavy smokers tend to consume a lot of coffee. In short, (D) weakens by attacking the correction made by the study about smoking habits. The study results from the stimulus is basically after the correction of smoking habits, but choice (D) attacks it by saying the people who drink this much coffee tends to be heavy smokers.
#20: the main focal point of the stimulus is that introduction of products should have "good understanding". The stimulus concludes by saying because of newly marketed AH are far from clear, there should be a reduction in the pace of new drugs that are being now tested. But in the first sentence of the stimulus, it states that "introduction of a new drug should be contingent upon good understanding of the product". (A) is correct because if the understanding of the new AH is better than the new drugs being tested; it is basically saying the new drugs being tested are even far from being understood, since the AH's understanding is currently far from clear.
Hope this helps.