圆弧螺旋齿轮的啮合印痕长度必须大于齿长的（）。
A．1/2
B．3/4
C．2/3
参考答案：C
If you don't learn something useful, you will feel sad ______ you are old. [ ]
A. what
B. when
C. which
D. why
坊木纹技法是（）应掌握的.
A.油漆工
B.木工
C.电工
D.水工
