In areas like the Central United States that get lots of tornadoes， there′s a common piece of celestial （天上的）fortune-telling that green skies mean a tornado is on its way． Research has shown that it isn′t quite as simple as that， but scientists have found that if you see a green sky， you should probably go inside． As far as most scientists can tell， the green skies around powerful thunderstorms are usually a combination of red sunsets and water droplets．
Daytime skies are blue because blue shorter wavelengths of visible light tend to bounce off （反 射）air molecules better than red longer wavelength light． So the blue light gets bounced all over the sky and looks like it's coming from everywhere． Meanwhile， around sunset sunlight travels through so much atmosphere that just about all the blue is bounced away from the horizon， leaving all those reds and oranges behind． The sunset might look exclusively （独有的）red and orange， but there's still some green and even blue light hidden in there， just far less than the other colors． But for us to notice the remaining green light， it needs to hit something that reflects green light much better than red． That's where water comes in．
Big tall threatening storm clouds are made of water droplets， and even though water is best at reflecting blue light， it can still reflect green pretty well， much better than reds and oranges， so under just the right conditions， the water in and around a cloud can bounce the green light hidden in the sunset right into our eyes making the sky look green．
Still， none of the conditions that can turn skies green are unique to the clouds that produce tornadoes． Sometimes they just lead to lots of rain， and maybe some hail． So if you see huge storm clouds rolling in and a green tint （色调）in the sky， it doesn't necessarily mean there′s about to be a tornado， but it does mean you should probably head indoors．



What can be the best title for the text？ ______

A. Why Do the Central United States Get Tornadoes
B. Do Green Skies Mean a Coming Tornado
C. A Common Piece of Celestial Fortune-telling
D. Green Skies Are Unique to Clouds

Why are the skies blue in the daytime？ ______

A. Because only blue light can be bounced in the daytime．
B. Because there is more blue light than light of other colors．
C. Because blue light is bounced all over the sky．
D. Because red light is weaker than blue light．

What does the underlined word "it" in paragraph 2 refer to？ ______

A. The sunset．
B. Red light．
C. Blue light．
D. Water．

What can we learn about from the text？ ______

A. Storm clouds are made of water droplets and green light．
B. Water can reflect red light as well as blue light．
C. There are only red and orange lights in the sunsets．
D. If you see a green sky， there will be a tornado．