单选题 The doctor (told) him that he (should take) this medicine (one) pill at (one) time.
【正确答案】 D
【答案解析】短语用法错误,应改为a。at a time“一次,每次”at one time“曾经”。本句表示一次吃一片药,而不是曾经吃一片药。