采用电话闭塞法时，相邻两站运营方向（）站界标之间为一个电话闭塞区段。
参考答案：头端
--- I’d like to buy a skirt like this.
--- OK. We can buy ________ one than this, but ________ this.
A．a good; not as cheap as
B．a better; as expensive as
C．a better; not as expensive as
D．a better; not so cheap as
理想的脑灌注显像剂应具备（）。
A.不能穿透血脑屏障
B.能穿透血脑屏障
C.在脑中滞留足够的时间
D.具有确定的脑区域分布
E.迅速从体内清除
