Sufficient vs necessary

I got the correct answer, but I don't see how it could be *sufficient* to, by itself, guaranty the the conclusion. However, I do see why it's *necessary* for the conclusion; it just seems like it needs to be combined with the other premise (in the passage) to arrive at the conclusion. How is the correct answer a super premise? If someone tells me that "The number of visits to hospital emergency rooms by heroin users is proportional to the incidence of heroin usage," how can I -just from that- conclude that "the use of heroin has risen"? I would also need to know the other premise (that "The number of hospital emergency room visits by heroin users grew") in order to conclude that overall heroin use has risen.