题目

His ears are highly ______ to any unusual sound in the machine, that is how he found the fault.   A. sense          B. sensitive         C. sensitively           D. insensitive 答案：B已知直线a，b是直线，α，β，γ是平面，给出下列命题：①a∥α，a∥β，α∩β=b，则a∥b；②a⊥γ，β⊥γ，则a∥β；③a⊥α，b⊥β，a⊥b，则α⊥β；④a∥β，β∥γ，a⊥α，则α⊥γ．其中正确命题的序号______．