LSAT-98-06-2**about if and only if**

I always have problem attacking this kind of question...can anyone please give me more detailed answer? thank you so much!!

23.    Politician: Nobody can deny that homelessness is a problem, yet there seems to be little agreement on how to solve it. One thing, however, is clear: ignoreing the problem will not make it go away. Only if the government steps in and provides the homess with housing will this problem disappear, and this necessitates increased taxation. For this reason, we should raise taxes.

Which one of the following is principles, if valid, most supports the politician's argument?

A) Only if a measure is required to solve a problem should it be adopted.

C) If a measure is required to solve a problem, then it should be adopted.

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C#...why not A?