单选题 Triangle ABC shown above is an isosceles triangle. Which of the following statements individually provide(s) sufficient additional information to determine the measure of angle B?

A、 The measure of angle A is 80°.
B、 The measure of angle C is 50°.
C、 The measure of angle A is 80° and the measure of angle C is 50°.

【正确答案】 C

【答案解析】An isosceles triangle has two congruent sides, and the angles opposite those sides are also congruent. In any triangle, the total of all three interior angles is 180 degrees. Because you do not know which sides are congruent, neither (A) nor (B) alone is sufficient to answer the question; angle B could be either 80° or 50°. However, if you take both statements together, as in statement (C), you know that angle B must equal 50°, because 180°-80°- 50°= 50°.