I just don’t understand ________ that prevents so many children at school from being as happy as one

might expect.[ ]

A. what it is          

B. what it does        

C. what is it          

D. why is it

关于个体身心发展的动因理论有（）。

A.内发论

B.外铄论

C.成熟机制

D.教育

E.多因素相互作用论