Suspecting the traveler of carrying drugs，the Customs official stopped him and________his suitcase，

but found nothing at all.[      ]

A．looked up

B．went through

C．searched for 

 D．referred to

真毛细血管不具有下列哪一项特点

A．管壁很薄、通透性大

B．血流很慢

C．微动脉阻力对血流控制的作用不大

D．是血液和组织液进行物质交换的场所

E．安静时，骨骼肌中大约有20%～35%的真毛细血管处于开放状态