题目

8、When and where to go for the on-salary holiday ______ yet. A. are not decided     B. have not been decided C. is not being decided    D. has not been decided 答案：D如图所示，用悬挂着的乒乓球接触正在发声的音叉，乒乓球会被多次弹开．这个实验是用来探究 A．声音能否在真空中传播          B．声音产生的原因 C．音调是否与频率有关            D．声音传播是否需要时间