单选题：二维矩阵推导
对二维矩阵matrix = [[1, 2, 3], [4, 5, 6], [7, 8, 9]]里每个元素计算平方值，以下哪条语句能够正确得到二维矩阵：
[[1, 4, 9], [16, 25, 36], [49, 64, 81]]
A.squares_matrix = [x**2 for row in matrix for x in row]
B.squares_matrix = [[x**2 for x in row] for row in matrix]
C.squares_matrix = [x**2 for x in matrix]
D.squares_matrix = [[x**2 for row in matrix] for x in row]
答案：B
[[1, 4, 9], [16, 25, 36], [49, 64, 81]]
A.squares_matrix = [x**2 for row in matrix for x in row]
B.squares_matrix = [[x**2 for x in row] for row in matrix]
C.squares_matrix = [x**2 for x in matrix]
D.squares_matrix = [[x**2 for row in matrix] for x in row]
答案：B
