题目

―After two weeks of tennis lessons, I think I finally managed to improve my backhand.―Like my mom always says: “         .”   A．Pride goes before a fall                             B．Think twice before you leap   C．It never rains but it pours                         D．Practice makes perfect 答案：D电流的符号是    ．它的主单位是    ，用    （字母）表示．