grammar - Answering a singular question with a plural answer

If somebody asks a question in the singular, how do we give a plural answer? Here's an example, with three different possible ways to answer. (The part of the answer that has the plural sense is marked in bold):

Q: What's the most important thing to you?

A1: The most important thing to me is my friends.

A2: The most important thing to me are my friends.

A3: The most important things to me are my friends.

I am a native speaker of British English. I prefer A1. An American told me she preferred A2. I suspect that a formal prescriptivist would recommend A3.

Is there any rhyme or reason to this? What should I recommend to a student learning English?

Answer

These are cleft sentences where there is a separation the grammatical and the logical subject, and both A1 and A2 make sense.

In A1 the verb agrees with the grammatical subject: The most important thing to me (singular).

In A2 the verb agrees with the logical subject: my friends (plural).

The odd one out is A3

"The most important thing to me" as in A1 and A2 implies a single item or a single set of items.

"The most important things to me" on the other hand implies a list of (sets of) items, but "my fiends" is not a list.