The train is running 120 kilometres an hour.A. at a speedB. at the speedC. at a speed ofD

问题补充：

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

答案：

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。