最后一句求解释！我大一的

Question

Answer 1

a = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]
s = 0
for c in a:
    # c: [1, 2, 3] [4, 5, 6] [7, 8, 9]
    for j in range(3):
        # j: 0 1 2  下标
        s += c[j]  # s = s + c[j]
print(s)