题目

.已知在中，三个内角A,B,C成等差数列，三边a,b,c成等比数列，则是                                    （  ） A直角三角形 B等边三角形 C等腰三角形 D等腰直角三角形 答案：B . The production of television commercials must be professional ________ they aim at promoting good service, or public education. A．when B．no matter C．if D．whether