21. Leaves are found on all kinds of trees, but they differ greatly ________ size and shape.
(A) on
(B) from
(C) by
(D) in

介词in表示“在......方面。句意：“人们发现树上长着各种各样的叶子，但它们的大小、形状大不相同。differ from与…..不同，与本题题意符合。