用反证法证明同一三角形中至少有两个锐角,证明时应假设______.
用反证法证明同一三角形中至少有两个锐角时,应先假设同一三角形中最多有一个锐角.
喷砂除锈后,金属结构表面粗糙度应在()的范围之内。
A、Ry40~100μm
B、Ry100~200μm
C、Ry400~600μm
D、Ry600~800μm
It was not until liberation that _________ to his hometown.
A.did he return
B.was he returned
C.he did return
D.he returned