a, based on cultivating such crops like corn and beans, the harvesting of fish and shellfish, and exploiting b, based on the cultivation of such crops as corn and beans, the harvesting of fish and seafood, and the exploitation of 刚刚发现的小细节,a中是fish and shellfish, 而b中是fish and seafood, fish不属于seafood的么?莫非是河鱼?否则fish and seafood不是一类的啊···求解···