答案：A

解析：

In order to find the value of x, you should first determine the measure of the angle that is located at point O in the right triangle. To determine this angle, you must calculate what fraction of the circle's area is unshaded. The radius r of the circle is 5 and its area isπ r^2, or 25π. The area of the shaded region is 20π, so the area of the unshaded region must be 5π. Therefore, the fraction of the circle's area that is unshaded is (5π)/(25π), or 1/5. A circle contains a total of three hundred and sixty degrees of arc, which means that 1/5 of 360 degrees, or 72 degrees, is the measure of the angle at point O in the unshaded region. Since you now know that two of the three angles in the triangle measure 72 degrees and 90 degrees and that the sum of the measures of the three angles is always 180 degrees, the third angle must measure 18 degrees. Therefore, x = 18.