You promised your friend a letter; you ought to ______ days ago.

A．write

B．be writing

C．have written

D．be written

能显示与整个脊柱相似的生物力学特性的最小功能单位称为（）

A.运动节段

B.活动节段

C.部分节段

D.最小节段

E.最短节段