常规保养中“三滤”包括（）、（）和（）。
参考答案：机滤；汽滤；空滤
At no time _____ was happening. [ ]
A. the President was aware of what
B. was the President aware of what
C. the President was aware that
D. was the Ptesident aware that
同步发电机励磁自动调节的作用是什么？
