单选题 At the moment the machine ______ at full speed.
A. ran B. was running C. had been running D. had run

【正确答案】 B
【答案解析】[解析] 本句意为：此时此刻，机器正在全速运转。 at the moment意为“此刻”，可以推知后面的句子应该选用进行时态。