— Listen! Someone is knocking at the door. Who can it be?
— It _______ be Tom. He said he would visit me this evening.
A：can't
B：need
C：must
D：mustn't
小张得了肠炎，医生将药物与生理盐水从他的前臂静脉输入，当药物作用于小肠时，经过的途径依次是（ ）
①肺动脉 ②肺静脉 ③上腔静脉 ④心脏 ⑤小肠毛细血管 ⑥肺部毛细血管 ⑦主动脉．
