IMO D

for k =odd \(a_k\) will be \(a_1\) =1 ; \(a_3\)=3 ; \(a_5\)=5 ; so on

for k=even \(a_k\)=-\(a_(k-1)\) will be \(a_2\)=-\(a_1\)=-1; \(a_4\)=-3 ; \(a_6\)=-5 :SO ON

SEQUENCE will be

1-1+3-3+5-5+7-7 ( when n is even sum=0 and when n=odd sum=n)

s1) for all n=odd sum=n and hence it is either +1,+3,+5,+7.....SO ON BUT ALL +VE

HENCE SUFF

s2)if \(a_n\) is +ve -----> n is odd and hence sum is +ve =n

hence suff

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