Yes, the answer is C. (2) tells us
indirectly that the triangle is a right triangle, and (2) tells us the non-right angles of the triangle.
patogzz Wrote:For why understand, the rule says that an angle inscribed in a semicircle is a right angle, but points A and B could be anywhere (not necessarily in the middle forming a semicircle).
The problem doesn't specifically say that angle ACB = 90 or that points A and B are exactly at opposite sides therefore I think the answer should be E.
You are right, the problem doesn't say
outright that A and B are exactly at opposite sides of the circle. However, we know that for all circles, Circumference = pi*d = 2*pi*r. Since statement (2) says that Circumference = 18pi, that is indirectly telling us that diameter = 18. From the picture, if AB is 18, then it is a diameter. That implies that angle ACB is 90 degrees.
This is typical of GMAT geometry problems--you often have to use a couple rules. the first rule lets you infer something, so label the diagram with that new inference. Then, you'll see something else that can be inferred from that new label with yet another rule. This is why drawing the picture on your own paper and adding dimensions, angles, labels, etc. as you go is so important.
