设Q为有理数集，a，b∈Q，定义映射f_a，b：Q→Q，x→ax+b，则f_a，b•f_c．d定义为Q到Q的映射：（f_a，b•f_c．d）（x）=f_a，b（f_c．d（x）），则（f_a，b•f_c．d）=（　　）

A．f_ac，bd

B．f_a+c，b+d

C．f_ac，ad+b

D．f_ab，cd

In my opinion, all Mr Li ______ good to his students in his class at present. He is very strict with them in their studies.

A．does does does

B．does does do

C．does do did

D．did do does