有如下程序段，则该程序段的时间复杂度为(   )

a = [0 for i in range(n+1)] for j in range(n+1)]

a[0][0]=1

for i in range(1,n+1):

    a[i][0]=1

    for j in range(1,i+1)

        a[i][i] = a[i-1][j-1]+a[i-1][j]

A、O(log2N)

B、O(N)

C、O(Nlog2N)

D、O(N2)