An operation is usually performed by a ______.
A.doctor
B.dentist
C.surgeon
D.scientist
答案:C
金刚石刀具既适合于黑色金属的精加工,又适合于有色金属的精加工。
空气中的含氧量低于()时,燃烧自行中止
A、10%
B、14%
C、16%