设等差数列{an}的公差d≠0，a1=4d，若ak是a1与a2k的等比中项，则K的值为（ ）．
3
It is required that each of the students chosen _____ good preparations ahead of time.
A．has to make
B．will make
C．should make
D．must make
心肺复苏的有效指征( )。
A．颈、股动脉可触及搏动
B．散大的瞳孔缩小
C．自主呼吸恢复
D．自行排大小便
E．上肢收缩压在8kPa(60mmHg)以上
