题目

某一交流的方向每秒钟改变100次，则它的周期和频率分别是（ ） A . 0.01s和50Hz B . 0.01s和100Hz C . 0.02s和50Hz D . 0.02s和100Hz 答案：C17、All the streets in Moscow were         with flags to welcome chairman Hu Jingtao.        A．furnished             B．supplied               C．decorated            D．declined