Data Sufficiency

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If sequence a_1,a_2,a_3,.........,a_n of n integers is such that ak=k if k is odd and a_k=-a_(k-1) if k is even. Is the sum of the terms in the sequence positive?
1) n is odd
2) a_n is positive

In the original condition there are multiple variables and since we need just as many equations, E is likely the answer. Using both 1) & 2) together, 1) =2) therefore D is likely the answer.

Assuming n=odd=3, a_1=1, a_2=-a_1=-1, a_3=3 and the last term is always left out. Therefore the the sum is positive. The answer is yes, and the condition is sufficient. Therefore the answer is D.


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