用电火花机床不能加工的形状或材料为()。
A. 盲孔
B. 圆孔
C. 上下异形
D. 淬火钢
The house _______ many years ago.
A. is built
B. was built
C. has been built
D. had been built
The article __________ many times.
A. has been written
B. was written
C. wrote
D. write
You ________ clean that room. It is very clean now.
A. must
B. have to
C. need to
D. don’t need to