下列代码的输出结果是（   ）。
ls=[[1,2,3],[4,5,6],[7,8,9]]
new_ls=[j for i in ls for j in i]
print(new_ls)
A、[[1, 2, 3], [4, 5, 6], [7, 8, 9]]
B、[1, 2, 3, 4, 5, 6, 7, 8, 9]
C、[[1, 2, 3], [1, 2, 3], [1, 2, 3], [4, 5, 6], [4, 5, 6], [4, 5, 6], [7, 8, 9], [7, 8, 9], [7, 8, 9]]
D、异常