单选题 —I'm sorry I stepped outside for a smoke. I was very tired.
—There is no ______ for this while you are on duty.

【正确答案】 B
【答案解析】[考点] 口语应用
[解析] excuse意为“托词，借口”；reason意为“理由”；explanation意为“解释”；cause意为“动机，原因”。