简述程序教学的基本原则。
参考答案：(1)小步子逻辑序列；
(2)要求学生作出积极反应；
(3)及时反馈；
(4)学生自定步调；
(5)低的错误率。
On his way home, he found a wallet ___ on the ground, which he picked up and ____ in his schoolbag.
A．lying, laid
B．laying, laid
C．laid, lay
D．lain, lay
简述谷物中水分存在的形式和干燥形式？
