急求 OG上的一个SC的问题。

mrj1104

28，Building on civilization that preceded them in coastal peru, the Mochica developed their own elaborate society , based on the cultiucation of such crops as corn and beans , the harvesting of fish and seafood, and the exploitation of other wild and domestic resources.
我想问下，为什么是the harvesting 而不是 the  harvest  ,  在字典上找不到harvesting 这个单词么。。。。。。大牛快来看下啊啊啊。。。