正方体的全面积为6，它的顶点都在球面上，则这个球的表面积是 [ ]
A．3π
B．4π
C．6π
D．8π
答案：A
Robert is said _______ abroad, but I don’t know what country he studied in.
A．to have studied
B．to study
C．to be studying
D．to have been studying
数控编程数控系统在执行N5 T0101 N10 G00 X50 Z60；程序时，（）。
A.在N5行换刀，N10行执行刀偏
B.在N5行执行刀偏后再换刀
C.在N5行换刀同时执行刀偏
D.在N5行换刀后再执行刀偏
