The country's railway system has launched online ticket-booking and a real-name ticket pur

问题补充：

The countrys railway system has launched online ticket-booking and a real-name ticket purchasing system, and requires passengers to provide their ID numbers when buying tickets.

A. which

B. that

C. what

D. this

答案：

A. which

(答案→)B. that

C. what

D. this

解析：本题考查代词。题干中and已是连词，不要其它连词。此处that是代词，用来指代前面句子的内容。