围绕本课标题 Sold out 票已售完（售票处的买票与退票）主题，AI 老师会出 若干个简单问题，
每个问题听完后，用 完整句子口头作答。
⚠️ 尽量说完整句子，不要只说一个词；优先用本课句型与词语：may begin at any moment / may have begun already / we've sold out / What a pity / Can I return these two tickets / at once / I might as well have them