— Do you regret________ up your opinion that every child should take part in service learning?

— No.On the contrary, I think it is very necessary. [ ]

A. bringing  

B. to bring

C. to be brought  

D. being brought

已知三棱柱ABC-A1B1C1的侧棱与底面边长都相等，A1在底面ABC内的射影为△ABC的中心，则AB1与底面ABC所成角的正弦值等于（ ）。

A． 13

B． 23

C．33

D． 23