如果想要将一个二维列表 matrix=[[1, 2],[3, 4],[5, 6]] 扁平化为 [1, 2, 3, 4, 5, 6]，应使用下列哪个列表推导式？

[item for item in matrix]

[item for row in matrix for item in row]

[row for row in matrix for item in row]

[row for item in matrix for item in row]