DNA分子中的某一个区段上有300个脱氧核糖和60个胞嘧啶,那么该区段胸腺嘧啶的数量是 [ ]
A.90
B.120
C.180
D.240
答案:A
Please turn towards me when you____________.so that I can hear what you are saying.
A.will speak
B.have spoken
C.are speaking
D.will be speaking
用刀具切除工件上一层多余的金属过程和形成已加工表面过程叫()过程。
A.金属切削
B.加工
C.走刀
D.运动