Let \left\{a_{k}\right\} be a sequence of integers such that a_{1}=1 and a_{m+n}=a_{m}+a_{n}+m n, for all positive integers m and n. Then a_{12} is
Answer Choices
A. 45
B. 56
C. 67
D. 78
E. 89
Let \left\{a_{k}\right\} be a sequence of integers such that a_{1}=1 and a_{m+n}=a_{m}+a_{n}+m n, for all positive integers m and n. Then a_{12} is
Answer Choices
A. 45
B. 56
C. 67
D. 78
E. 89