这道题选it还是one

请看下面一道题：
I want very much to buy a colour TV set, but I can’t afford _________.
A. it                     B. one                   C. this                    D. that
答案选B，但容易误选A。it 和 one 的区别可简单地概括为：it = the + 名词，one = a + 名词。如：I haven’t any dictionaries, and I want to borrow one. 我没有词典，我想借一本。(one = a dictionary)I have two dictionaries, and I can lend one to you. 我有两本词典，我可以借一本给你。(one = a dictionary)
I have a dictionary, and I can lend it to you. 我有一本词典，我可以把它借给你。(it = the dictionary)

我有一本词典，我可以把它借给你。(it = the dictionary) (from 根据上面一题的语境，句子后半截显然应该是 … but I can’t afford a colour TV set，而不是 but I can’t afford the colour TV set，所以答案选B。