Q. 16 is a challenging question. The argument is claiming that Beethoven's deafness was caused by a VD, which would be confirmed by finding mercury in his hair. After all, people in Beethoven's day would ingest mercury to treat VD.
However, how do we know people didn't ingest mercury for other reasons? If you find mercury in Beethoven's hair, does it HAVE to be because he had a VD. Of course not. But, the author is assuming this to be the case. So, he assumes that only people with VD would have ingested mercury -- people without a VD wouldn't have ingested it. (B) gets in line with that by saying that some people didn't ingest mercury.
Because this question is looking for a required assumption, consider what would happen to the argument if (B) was false. What if everybody in Beethoven's time ingested mercury (say, mercury was found in the water)? Then, the argument wouldn't make sense at all. Why would the author assume the mercury was there for a VD? The author MUST assume that mercury wasn't ingested by everybody -- exactly what (B) says.
This doesn't make this question easy by any stretch of the imagination. But (B) is the only acceptable answer in the bunch.
Q. 19 is not nearly as bad -- once you catch the flaw. The conclusion says that some of the nine cancelled FLIGHTS must be caused by something other than mechanical problems. However, the evidence is that mechanical problems would only really affect one or two PLANES. The author confuses # of planes with # of flights.
What if those nine flights were all serviced by one plane (like a Boston - NYC shuttle)? If it was just one plane, wouldn't mechanical problems seem plausible?
The author definitely assumes (as you may have naturally done, too) that those nine cancelled flights required nine planes -- or at least more than the two planes that would be expected to have mechanical problems. That's what (A) says -- more than two planes were schedule for those nine flights. Adding that to the logic of the argument, since mechanical problems would only afflict one or two planes, the remaining planes would probably have something else wrong.
Unfortunately, I had Q. 16 in front of me, I know Q. 19 very well, but Q. 24 is MIA for me. If no one else responds, I'll try to post something else a little later.
Hope this helps.