方程x2=4x的解 ______．
原方程变为
x2-4x=0
x（x-4）=0
解得x1=0，x2=4．
— ______ that he picked up his uncle at the airport?
— It was at 6:00 in the afternoon ______ he got off work. [ ]
A. Was it when; that
B. When was it; that
C. Was it when ; when
D. When was it; when
大气中氧的含量超过（）时，会使火星发展成为火焰。
A、21％
B、27％
C、35％
D、40％
when ; when