题目

A poet and artist ____ coming to our school to give us students a lecture about Chinese literature tomorrow afternoon .    A. was                           B. are                    C. is                   D. were   答案：C已知正三棱柱ABC—A1B1C1中,?D是AB的中点,DC=DA1=a,(1)求异面直线BC1与A1D所成的角;(2)求二面角DA1CA的大小;(3)求点C1到平面A1DC的距离.