一般用途的阀杆螺母的磨损以不超过梯形螺纹厚度的（）为准，超过者应更换。
A、1／2；
B、1／3；
C、1／4；
D、1／5。
参考答案：B
—Why didn’t you buy the gold ring?
—I ______ but I didn’t have the money.
A．would have
B．bought
C．would like to
D．had bought
Suppose a given binary tree has 10 left nodes，then the number of nodes with degree of 2 is
A．12
B．11
C．9
D．Indeterminable
