Tornadoes, whirlwinds, and water-sprouts are rotating funnel cloud air masses of small dia

meter. They are differentiated by the intensity of their rotation and by the surfaces that they traverse. Though tornadoes and whirlwinds both travel over land masses, whirlwinds are atmospheric systems smaller than tornadoes. Water-sprouts are tornadoes that form. or pass over a water surface. A tornado is a powerful vortex or "twister" whose rotational speeds are estimated to be near 300 miles per hour. The first visible indication of tornado development is usually a funnel cloud. As this funnel clips earthward, it becomes darker because of the debris that is forced into its intensifying vortex. Some tornadoes give no visible warning until their destruction strikes the unsuspecting victim. Tornadoes often occur in groups, and several twisters sometimes descend from the same cloud base. The forward speed of a tornado is normally 30 to 40 miles per hour. In the short time that it takes to pass, a tornado can cause fantastic destruction. There have been cases reported in which blades of straw were embedded in fence posts.

How are tornadoes distinguished from whirlwinds？

A．Whirlwinds rotate in a different direction than tornadoes do.

B．Tornadoes travel over land and whirlwinds over water.

C．Tornadoes are larger than whirlwinds.

D．Only tornadoes are funnel-shaped.