我国学制体系由哪几部分构成（）（）（）。
参考答案：学前教育系统；初等教育系统；高等教育系统
已知am=6 ，an=2 ，求a2m-3n的值．
--- Mum, where is my toy cowboy? I________it under the bed.
--- It_____inside your toy bucket. Remember, that’s where all your toys are supposed to go.
A．have put; is
B．put; is
C．have put; was
D．put; was
