刀具的选择主要取决于工件的外形结构、工件的材料加工性能及（）等因素。
A、加工设备
B、加工余量
C、加工精度
D、表面的粗糙度要求
参考答案：D
—What did your math teacher say ?
— She said I was ____________.
A．work hard
B．working hardly
C．hard-working
D．hard work
What ______ useful book it is! [ ]
A. a
B. an
C. the
D. /
