Flights are often canceled for many different reasons，including snow，clouds，or even a bird getting caught in an airplane's engine.But you never think that too much sunshine would stop a plane from taking off.
   Unfortunately for passengers in Phoenix，US this summer，their travel plans were ruined when more than 40 flights of the local airport were canceled after temperatures went to 48℃.This is the maximum temperature that many smaller planes are allowed to fly at.
   So what is it that makes flying in extreme heat so dangerous？The answer lies in how thick the air is.
   "As air warms up，it expands（膨脹），and there's fewer molecules（分子） under your wings，" a professor at Embry-Riddle Aeronautical University in the US said.
   When the temperature is high and the air becomes thin， "You need more of everything.You need more thrust（推力） to take off.You need more distance to take off.You need more distance to land.It's too much for some planes，" the professor explained.
   The US' Southwest，where Phoenix is located，is experiencing an unusually hot summer.This June was its third-hottest on record.Up to 10 people are thought to have died as a result of the hot weather.
   The unexpected heat took airlines by surprise，causing them to cancel all flights running in smaller planes for the safety of their passengers.
   But what about cities whose temperatures are high every summer？
   Many airlines in hot Gulf countries such as the United Arb Emirates operate many flights at night or in the early morning to avoid the problem of thin air.They also mainly use larger planes，which are able to operate in greater heat than smaller jets.
   After the temperature in Phoenix continued to climb over 40℃ throughout the rest of the summer，maybe now would be a good time for airlines to buy bigger planes in time for next year's heat wave.
（1）Which of the following reasons of canceling flights is NOT mentioned in the passage？

C

C

A．Snow.
B．Clouds.
C．Sandstorm.
D．Birds.
（2）When the weather is too hot，

B

B

.
A．planes are easier to breakdown
B．planes need more power and distance to take off
C．there is much more air under the wings of a plane
D．smaller planes are safer for passengers
（3）How do airlines in Gulf countries deal with high temperatures？

D

D

A．They operate less flights in summer.
B．They mainly use small planes.
C．They build long runways for planes.
D．They operate fewer flights at noon.
（4）What does the underlined word "maximum" in the second paragraph mean？

A

A

A．highest
B．lowest
C．best
D．worst
（5）What's the article mainly about？

D

D

A．Flight accidents caused by extreme heat.
B．Perfect flying condition for airplane.
C．The latest change of flight safety rules.
D．How hot weather influences flights.