正常情况下微笑时上切牙暴露比例和下切牙暴露比例分别为

A．1/3、1/3
B．1/2、1/2
C．1/2、1/3
D．2/3、1/2
E．2/3、2/3

In my opinion, parents and teachers are supposed to set an example of good behavior to children _____ they are role models to them.

A．although

B．even if

C．unless

D．as