The purpose of the proposed law requiring a doctor's prescription for

To get this one, it's important to acknowledge the strategy the author is using. They aren't saying that hypodermic needles aren't dangerous, not are they arguing that the law wouldn't work to make people safer. They're simply bringing up a different case. "Hey, this is dangerous, and no one wants to ban it. So we shouldn't ban this other dangerous thing!" This is a form of reduction ad absurdum, in which we try to make the logical consequences of an opponent's argument as preposterous as possible: "If you believe X, you also have to believe some crazier thing that you don't want to agree with. Therefore, discard X."

There was actually a very famous statement like this. When asked about the harmful effects of cigarettes, a tobacco executive said "Anything can be considered harmful. Applesauce is harmful if you get too much of it." Naturally, the interviewer came back with what you are probably thinking: People aren't dying from applesauce, so it's a bad parallel!

It's the same thing with this needle argument. The premise does say the knitting needles CAN be lethal, so we have to accept that. But are they actually causing deaths? If not, then maybe the parallel is a poor one, since it has been established in the argument that hypodermics ARE causing deaths. A improves the argument by saying that knitting needles do indeed cause deaths, so it makes at least a little sense for the author to ask why we want to limit access to hypodermics but not to knitting needles.

C, on the other hand, addresses whether the law would curb dangerous behavior. If the author were objecting to the law on the grounds that it would be ineffective, this would be a good strengthener. But since the author is objecting in a completely different way, C doesn't matter.