按要求完成下列各题，每空一词。

1.  Why not join an English club to practice speaking English?(改为同义句)

     _______ _______ _______join an English club to practice speaking English?

2.  If you want to improve English, you should practice writing a lot.(改为同义句)

     Lots of writing practice_______ _______you improve your English.

3.  The girl is so young that she can't ride a bicycle.(改为同义句)

     The girl is_______young_______ ride a bicycle.

4.  Tom don't know how to prepare the mid-exam.(改为同义句)

     Tom don't know_______ _______ _______prepare the mid-exam.

胚内中胚层的来源是：（）。
原条头端的结构是：（）。

A.外胚层

B.原结

C.原条

D.原沟

E.口咽膜