When the H1N1 virus____ at first in Mexico, no one had the least idea that it would spread so rapidly throughout the world.

A．broke off

B．broke down

C．broke out

D．broke away

相撞系指铁路机车车辆在运行过程中与行人、机动车、非机动车、牲畜及其他障碍物相互（），造成人员伤亡、设备设施损坏。