The train is running 120 kilometres an hour.
A. at a speed
B. at the speed
C. at a speed of
D. on a speed of
网友回答
A. at a speed
B. at the speed
(答案→)C. at a speed of
D. on a speed of
解析：考查固定搭配。句意：那辆火车以每小时120公里的速度行驶。at“以，按照......”，at a speed of以......的速度。故选C。