阅读理解。
Tornadoes are nature's most violent storms, which can cause deaths and destroy a neighborhood in seconds. A tornado appears as a funnel-shaped (漏斗状) cloud from a thunderstorm to the ground with whirling (飞快移动) winds that can reach 300 miles per hour. Damage paths can be over one mile wide and 50 miles long. Every state is at some risk from this disaster.
Some tornadoes can be seen clearly, while rain or nearby low-hanging clouds obscure others. At many times, tornadoes develop so rapidly that little, if any, advance warning is possible.
Before a tornado hits, the air may become very still. A cloud of debris (散片) can mark the location of a tornado even if a funnel can not be seen. Tornadoes generally occur near the edge of a thunderstorm, when the strong wind may die down. It is not uncommon to see clear, sunlit skies behind a tornado.
The following are facts about tornadoes:
The average forward speed of a tornado is 30 MPH, but may also vary from still to 70 MPH. Debris is picked up or a cloud forms in the funnel.
The average tornado moves from southwest to northeast, but tornadoes have been known to move in any direction. Tornadoes are most often reported east of the Rocky Mountains during spring and summer months.
Tornadoes can happen together with tropical storms and hurricanes as they move onto land. Waterspouts are tornadoes that form over water.
The tornado season in the southern states of America is March through May; in the northern states, it is late spring through early summer. Tornadoes are most likely to occur between 3 p.m. and 9 p.m., but can occur at any time.
（1）From the passage, where may tornadoes come from?
A.Hurricanes.
B.Great heat.
C.Thunderstorms.
D.Sandstorms.
（2）What does the underlined word “obscure” probably mean?
A.Hide.
B.Reduce.
C.Produce.
D.Raise.
（3）When a tornado happens, ________.
A.it often reaches the speed of 70 MPH
B.it can only be seen in the late of the day
C.it may form on the surface of sea water
D.it only moves from southwest to northeast
（4）The passage makes us believe that ________.
A.tornadoes are mostly seen in the southern states of America
B.warnings about tornadoes are well developed in many countries
C.of all the natural disasters, tornadoes often cause the biggest damage
D.tornadoes may strike so quickly that there is often little or no warning