If the radius of the circle circumscribing a triangle is...

Hello AAPL.

Let's take a look at your question.

The image represents the problem. All the red lines have the same length (equal to the radius). Then, the red triangle is equilateral. Therefore, its angles have the same measure 60º.

Now, using the Central Angle Theorem, we can get that A=2*B, that is to say, $$60º=2\cdot B\ \Leftrightarrow\ \ B=30º.$$ This is why the correct answer is the option [spoiler]A=30[/spoiler].

I hope this answer can help you.

I'm available if you'd like a follow-up.

Regards.