The OA from the answer key is C. I have been thinking about this too. I have been doing many CR and noticed that some of the answers are very subtle.
I will TRY to come up with some rationalization on this OA.
I choose C because in the latter part of the sentence, (after 'but') vitamin deficiency lacks a proper reference to the subject (pregnant women). The word 'their' after the second vitamin deficiency could mean preganant or the rest of the population.
I discarded B since the word 'pregnant' implies a higher need for nutrients and thus does not need additional clarification as to why preg. women need more vitamins. I thought it was logical to assume that preg. women need more vitamins.