题目

In choosing a friend, one should be very careful. A good friend can help you study. You can have fun together and make each other happy. Sometimes you will meet fair-weather friends. They will be with you only if you have money or luck, but when you are down, they will run away. How do I know when I have found a good friend? I look for certain qualities of characteristic, especially understanding, honesty and reliability (可靠). Above all else, I look for understanding in a friend. A good friend tries to understand how another person is feeling. He is not quick to give his opinion. Instead,已知实数abc≠0，则“线性方程组有无穷多组解”是“a、b、c成等比数列”的（ ）A．充分非必要条件B．必要非充分条件C．充要条件D．既不充分也不必要条件