单选题 Visitors can choose a time and date when they purchase tickets online and must arrive within a half hour of that time.
【正确答案】 C
【答案解析】本题的关键在于理清句子结构。本句主语是Visitors,and连接并列谓语can choose和must arrive,when引导时间状语从句,修饰第一个谓语can choose。