一、语法选择（每小题 1 分，共 15 分）
阅读下面短文，按照句子结构的语法性和上下文连贯的要求，从 1-15 各题所给的 A、B、C 和 D 四个选项中，选出最佳选项。
John and Mary had two lovely children. Once they were asked to go on a business trip. So they found a babysitter to take care 1 the children.
A week later, as they drove into their hometown, they noticed smoke and went to see 2 . A house was on fire! Mary said, “Well, the house isn’t 3 . Let’s go.”
