---What were you doing when Tom came to see you?

---I had just put on my overcoat and_____ to visit my friends.

A．left

B．was left

C．was leaving

D．had left

下列关于真毛细血管的特点的叙述不正确的是（）

A.管壁很薄

B.管壁的通透性大

C.血流缓慢

D.周期性收缩和舒张

E.管壁内皮细胞有吞饮功能