单选题：Given a 12$$\times$$12 symmetric matrix $$M$$. If the upper tri

Given a 12$$\times$$12 symmetric matrix $$M$$. If the upper triangular entries $$m_{i,j}$$ ($$1\le i\le j\le 12$$) of $$M$$ are stored row by row in a 1-dimentional array `N` (in C programming language). What is the index of $$m_{6,6}$$ in `N`? @[A](2)

A. 50
B. 51
C. 55
D. 66




A.50
B.51
C.55
D.66


答案：A