Tell me what's the len(A) and len(A[0]) ? I know len but i dont understand the A[0] in this code

Question

A = [[12,7,3],
[4,5,6],
[7,8,9]]

B = [[5,8,1],
[6,7,3],
[4,5,9]]

result = [[0,0,0],
[0,0,0],
[0,0,0]]

for k in range(len(A)):
for j in range(len(A[0])):
result[k][j] = A[k][j]+B[k][j]

Answer 1

A[0] is referring to [12,7,3]

Answer 2

len (A [0]) indicates that it is taking the length of the first element in the array, so the loop starts at 0 through 3, which is the length of the first element.
A[0] = [12, 7, 3]