What type should I assign if there are multiple possibilities?

I have two classes Class A and Class B. I have made a query, say repository.findById. This query may return output which may be of type A or type B.

Now, class B has all the fields which are in class A and some additional fields.

I know I can write like:

B res = repository.findById;

But writing like this is can be misleading as Class A and B are conceptually very different.

So basically the problem is

? res =repository.findById;

where ? can be A or B.

How should I solve this problem?