The sum of the even numbers between 1 and k is 79*80

The sum of the even numbers between 1 and k is 79*80, where k is an odd number, then k=?

(A) 79
(B) 80
(C) 81
(D) 157
(E) 159 






Sn=nA1+[n(n-1)d]/2

n=(k-1)/2, A1=2, d=2 ===>

2n+n(n-1)=n(n+1) ===>

[(k-1)/2]*[(k-1+2)/2]=(k-1)*(k+1)/4 ====>

(k-1)*(k+1)=79*2*80*2 ===>

k=159