根据对话内容填空（每空1分，共5分）

(Kangkang is booking train tickets to Mount Tai.)

Clerk:    Hello, Beijing Railway Station. Can I help you?

Kangkang: Yes, I’d like to book some tickets to Mount Tai    小题1: March 13^th.

Clerk:    OK. The train leaves at 11:45 a.m. and    小题2: at Taishan Railway

Station at 6:44 p.m.

Kangkang: Great. And what about the   小题3: ?

Clerk:    We have tickets at ￥145 for the hard sleeper and ￥224 for the soft sleeper. What _  小题4: do you want?

Kangkang: I’d like to book 21 hard sleeper tickets.

Clerk::    Please    小题5: for the tickets before 5:30 p.m. May I have your name and telephone number, please?

Kangkang: Sure. I’m Li Weikang and my telephone number is 8250-2448. Thank you.

Clerk:    You’re welcome.

药品只有在有效期限内使用，才能发挥药品的作用。这体现了药品的（）

A.有效性

B.安全性

C.专用性

D.时效性