Our company have been interviewing for a C# position recently. One of the questions we ask is "can you explain the difference between an abstract class and an interface". I've been to many interviews myself where this has been asked, and it feels to me like it should be an easy 30 second answer before moving on to something more interesting

However, all of our recent candidates responses have been "I don't know, I only use interfaces because they're better," which you could argue the case for but doesn't really answer the question adequately.

Are questions like this useful to determine a candidate's skill in an interview?

If not, what would be some better questions to ask?