Must be true. If a lake's water is replaced more often than once every six months, then it's not a candidate for liming. So if a lake is a candidate for liming, that implies its water is replaced only once every six months or less often.

Must be true. If a lake uses liming, then it must be treated periodically.

Could be false (and therefore correct on a Must Be True EXCEPT question). As shown below, we know what must be true if a lake uses liming. But we don't know what must be true if a lake does not use liming. So it's possible that lakes that do not receive liming, but in which water is replaced frequently, are less likely to be harmed by acid rain than those in which water is replaced infrequently.

Must be true. The stimulus tells us that liming can restore the health of some lakes in which life has already been harmed by acid rain.

Must be true. The stimulus tells us that a lake is not a candidate for liming if the water is replaced more often than once every six months because of the cost involved.