循环水的PH应控制在（）。
参考答案：7-9.2
如图所示，L1为入射光线，L2为反射光线，确定平面镜的位置．
The vocabulary and grammatical differences between British and American English are so trivial and few as hardly （）.
A．noticed
B．to be noticed
C．being noticed
D．to notice
