胸膜腔内压的负值大小取决 A．肺泡上皮细胞 B．肺间质 C．毛细血管基膜和内皮细胞

I think____________ necessary _____________ to take more exercise.

A．it, for Jim and me

B．it’s, to Jim and me

C．that , for Jim and I

D．that’s, to Jim and me

便秘的基本病机是（）

A.肠胃不和

B.肝气郁结

C.湿热下注

D.肺失开合

E.大肠传导失常

We shall  never give up ______ for even one day.

A．studying

B．study

C．to study

D．studies

尺寸链的特点是它具有封闭性和制约性。