单选题 ______ entering the house, he immediately found something strange.
[A] At [B] In [C] On [D] For

【正确答案】 C

【答案解析】[解析] 本题考查介词应用。本题句意为：一走进房子．他就立刻发现奇怪的地方。
[知识点] on/upon doing something表示“一…，就…”。例如：On leaving the airport，he phoned his family．他一离开机场就给家人打电话。