Why's choice B right?

I get that choice B is true, but why does it prove that the center of the inner circle is A?

@tidyboar Here's the thing choice B doesn't technically "prove" that it's the center, but, since the problem already tells you that the center of one of the arcs is at A and the other center is at the midpoint of BC, you know it has to be one of them. Basically, choice B says that the arc having a center at A is a reasonable answer given the diagram. And clearly, it can't be the midpoint of BC. Again, not a techincal proof, but it's a good strategy to use logic and make sense of the problem. Being able to judge whether or not an answer makes sense is a great skill to have!

@quacker88 Oh okay, thanks!