以认知为主线的学前游戏的发展分为以下几个阶段？ （ ）
A．虚构性水平
B．合作性水平
C．规则性水平
D．象征性水平
E．感觉运动性水平
参考答案：D,E
— He can't find his hat. Have you seen it?
— Yes, of course. I _____ it on his bed just now.[ ]
A. saw
B. have seen
C. had seen
D. to see
据丁颖的研究，我国的栽培稻可分为（）和（）2个亚种。
