In both these dialogues, the first response ( "Yes, I do") is the answer to the question "Do you like bananas?". The second response ( "But I don't .....") is the answer to an unspoken offer. The difference between the two dialogues lies in the number of bananas being offered. A Do you like bananas? Yes, I do. But I don't want one. In this dialogue, we understand that one person is offering A BANANA to the other person. That's why the response is 'one'. 'One' is the pronoun corresponding the singular indefinite article 'a'. B Do you like bananas? Yes, I do. But I don't want any. In this dialogue, we understand that one person is offering SOME BANANAS to the other person. That's why the response contains the word 'any'. The pronoun 'any' in a negative sentence is the opposite of 'some'. Do you want an apple? No, I don't want one. Do you want some/any apples? No, I don't want any.

In practice, the meanings are the same in this context. Do you want a banana? I don't want one (= a banana). Do you want any bananas? I don't want any (= any bananas - I don't want one, or two, or three, etc. of them).

"Do you like bananas?" refers to the banana "family" in general and includes every type of banana, for example big, small, ripe, not-so-ripe etcetera. "Would you like a banana?" is the question you would ask. the answer would then be "No, I don't want one." or more politely- "No thank you, not right now."

1.what are the differences between A and B? First, both are interchangeably used. But if you are looking for difference in nuances: A is stating he does not want specifically one but leaves open the possibility for portions thereof or even perhaps multiple bananas. A can plausibly be continued by: "Ok, but just have half." or "Ok, then have two." However, in most situations the intention is clear that A does not want half, one, two or any number of bananas. B specifies that he has no interest in bits, bites or portions of a or any bananas. 2.why does answer A use "one" , not use "ones"? "bananas" is plural. A is stating how many bananas he does not want. Generally regardless of how many he is offered the response is "I don't want one." However, if someone offered "How about two bananas?" the response could be "No thanks, I don't want two". "One" is a cardinal number. You are counting bananas; one, two, three, etc. We don't count using "ones" or "two's" for that matter.