the machine begins to work, you can’t stop it .A. While; soonB. For; right awayC. Once;

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A. While; soon
B. For; right away
(答案→)C. Once; at once
D. Until; immediately
解析：第一空考查连词：while虽然，for因为，once一旦until直到，第二空考查副词，soon很快，right away立刻，at once立刻，immediately立刻，句意是：一旦机器开始工作，你就不能让它停下。选C。