I could not do this in 2mins. picked a wrong strategy.. damn

Sn = 1/n - 1/(n+1)

Sum of first k terms = S1 + S2 ... Sk = 1/1 -1/2 + 1/2 - 1/3 ... + 1/(k-1) - 1/k + 1/k - 1(k+1) = 1 - 1/(k+1) = k/(k+1)

St#1 : k>10 , assume k=11, then Sum = 11/12, which is > 9/10, (also subsequent sums for k = 12, 13, 14 etc. will be greater that 9/10). Suff. there possible options AD

St #2: k<19, assume k=2, then Sum = 2/3, which is < 9/10 and we already saw that for k=11, sum > 9/10 so not suff. eliminate D => A.

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Looking for Kudos