I don't think you need a diagram. It definitely tests formal logic. "Most" means 1-99, so the first sentence, hypothetically, can say that 99 people out of 100 gains weight from antidepresents. The second statement says that dieting can negate this relationship but "some" or "at least 1" must always gain weight.So even if 98 people can use antidepresents and not gain weight, the second sentence assures that at least one must always gain weight no matter what. Hence choice C for the MBT question.