— Would you like to share your opinion with us?  

— Ok. Mary is studying harder than before, ____ everyone can see.

A．because

B．which

C．that

D．as

下列叙述中不准确的是（　　）

A．毛细血管连通动脉和静脉

B．动脉中只有动脉血

C．动脉管壁弹性大

D．静脉管壁薄