539.Leaves are found on all kinds of trees,but they differ greatly __________ size and shape.
A.on
B.from
C.by
D.in
540.Scientists are convinced _______ the positive effect of laughter _____ physical and mental health.
A.of;at
B.by;in
C.of;on
D.on;at
539.答案是D;differ in 在…方面不同；differ from 与…不同。
540.答案是C;be convinced of sth.坚信;the effect on sth.对于…的影响。
