题目

Our car equipped with a GPS, we had no trouble ______ the address of the headquarters of the United Nations.   A. finding        B. to find          C. found        D. to have been finding 答案：A如图曲轴上悬挂一弹簧振子，摇动摇把，曲轴可以带动弹簧振子上下振动，开始时不转动摇把，让振子自由振动，测得其频率为2Hz，然后匀速转动摇把，转速240转/分，当振子振动稳定时，它的振动周期为(   )   A．0.5s           B．0.25s            C．2s           D．4s