单选题 The mother would _____ her son doing his music practice if he could finish his assignment before supper.无

【正确答案】 C

【答案解析】句意：如果儿子能在晚饭前完成任务，妈妈就准许他暂停音乐练习。 let off准许……暂停工作。 let down使……失望。 let alone更不用说。 let out放出。因此，本题的正确答案为C。