第4题: [单项选择]_____ his work, he would do it till it was finished. A. Once Charles would start B. Once Charles started C. Since Charles would start D. Since Charles started 参考答案:B 答案解析:[解析] once意为“一旦……就……”,主句为将来时,时间状语从句不用将来时,而用现在时或过去时表示将来或过去要发生的动作,故选B。since的意思是“自从”或“由于”,与题意不符。