Hi,
Some people claim that exercise is a waste of time.
The author says the people only claim that because they are lazy.
In other words, the author's conclusion is that exercise is not a waste of time (and people who think it is a waste of time are lazy).
Now, in order to weaken the argument, we need a fact that makes the conclusion less likely to come true. Because the author is arguing that exercise isn't a waste of time, we need a choice that says exercise IS a waste of time. That's what choice B says. (A choice that suggested exercise IS NOT healthy would have also weakened the argument).
If it is the second one , should not C be the answer as it shows: it's not that exercise causes health, but it is that only healthy people do exercise.
First of all choice C deals with "likely" not "only". But that's minor; let's set that aside.
If only healthy people do exercise, doesn't that mean that NO unhealthy person exercises? Or that ALL unhealthy people DON'T exercise. If only healthy people exercise, then isn't the author's argument strengthened rather than weakened? Choice C is a 180.
Kaplan Teacher in Toronto