单选题：We can tell that there must be a lot of redundant calculations d

We can tell that there must be a lot of redundant calculations during the exhaustive search for the matrix multiplication problem, because the search work load is the Catalan number, yet there are only ___ different sub-problems $$M_{ij}$$.



A.$$O(N)$$
B.$$O(N^2)$$
C.$$O(N^3)$$
D.$$O(N^4)$$


答案：B