识别某种被子植物，首先要知道它属于[ ]
A．纲
B．科
C．属
D．种
答案：D
I won't tell the student the answer to the math problem until he _____ on it for more than an hour.[ ]
A. has been working
B. will have worked
C. will have been working
D. had worked
A．mgtgθ
B．mgsinθ
C．μmg
D．μmg
mgtgθ
mgsinθ