关于桩冠根管预备的要求，正确的是（）

A.根管口呈喇叭状

B.根管壁光滑、无倒凹

C.根管预备长度约为根长的2／3～3／4

D.根管粗度为根径的1／2

E.根管预备呈锥形

句型转换。

1. I have to stay at home. (改为否定句)

    I _____  _____ _____ stay at home.

2. Linda is taller than Jenny. (用the three girls改写句子)

    Linda is _____ _____ _____ the three girls.

3. David studied Chinese as hard as Mike .(改为否定句)

    David _____ _____ Chinese _____ hard as Mike.

4. The students saw several meaningful films last month. (改为被动语态)

    Several meaningful films _____ _____ _____ the students last month.

5. What a beautiful picture this is! (改为同义句)

     _____ _____ _____ picture this is!