Thousands who suffer heart attacks

The conclusion: The new agent could save the lives of many people who would otherwise join this group of heart attack victims.

The evidence: Thousands who suffer heart attacks each year die before reaching a hospital or clinic where they can benefit from the drugs that dissolve clots in coronary arteries. The Food and Drug Administration recently approved a new blood clot dissolving agent,

The assumption: The new agent will help people before they get to the hospital.

As is often the case, we can weaken the argument by attacking the assumption. Choice A does that. The author never said that the new agent works on people before they get to the hospital. If it has to be administered by a team of doctors in a hospital or clinic to work in time, it won't do what the conclusion says. Yes, it might be a wonderful drug, but it won't, as the conclusion says, save the lives of people in "this group" of heart attack victims -- the people who die on the way.

Choice B simply restates the evidence.
Choice C actually strengthens the conclusion. It provides that missing evidence (assumption) discussed above.
Choice D brings in irrelevant material. These other drugs don't matter.
Choice E says that the drug is not perfect, but that's not enough to say it won't save lives.