GMAT Sentence Correction (SC) | Re: During hot summer days to reduce the effect of sun s

The end of answer C is correct, and the end of B is not, since it changes the meaning. This is a bit of a subtle point, but if you look at B, it says:

"allowing the skin time to build a protective layer and to fight the radiation"

which, inserting the implied words, means:

"allowing the skin time to build a protective layer, and allowing the skin time to fight the radiation"

But the skin doesn't need to be allowed time to fight the radiation -- the skin needs the protective