— Sorry, Professor Smith. I didn't finish the assignment yesterday.

— Oh, you ________ have done it as yesterday was the deadline.[ ]

 A. must  

B. mustn't

C. should  

D. shouldn't

求空间曲线L：x=3lnθ，y=2sinθ，z=θ，(-∞＜θ＜+∞)在点P(3lnπ，0，π)处的法平面方程及切线．