Tom is one of _________ students in my class .Yes . He usually prefers _________ at home to playing outside .
A:the most hard-working , studying
B:more hard-working , to study
C:the most hard-working , to study
D:more hard-working , studying
在梯形ABCD中,AB∥CD,E,F分别是AD,BC的中点,且EF=2 cm,则AB+CD等于( )