B,
此处fish为可数名词,复数为本身.
that和it 表示特指.that与所指名词为同类,但不是同一个,而it 与所指名词为同一个.
I can't find my hat.I think I must buy one.(不定)
我找不到我的帽子了.我想我该去买一顶.
The hat you bought is bigger than that I bought.(同类但不同个)
你买的那顶帽子比我买的大.
I can't find my hat.I don' t know where I put it.( 同一物)
我找不到我的帽子.我不知道我把它放在哪了.