【单选题】

In the summer of 1978 anEnglish farmer was driving his tractor through a field of wheat when he discovered that some of his wheat was lying flat on the grounD、The flattened wheat formed a circle about six meters across.Around this circle were four smaller circles of flattened wheat. The five circles were in a formation like five dots.During the following years, farmers inEngland found the strange circles in their fields more and more often.

The circles are called "crop circles" because they appear in the fields of grain--usually wheat or corn. The grain in the circles lies flat on the ground but is never broken; it continues to grow, and farmers can later harvest it. Farmers always discover the crop circles in the morning, so the circles probably form at night. They appear only in the months from May to September.
At first, people thought that the circles were a hoax. Probably young people were making them as a joke, or farmers were making them to attract tourists. To prove that the circles were a hoax, people tried to make circles exactly like the ones that farmers had founD、They couldn’t do it. They couldn’t enter a field of grain without leaving tracks, and they couldn’t flatten the grain without breaking it.
Many people believe that beings from outer space are making the circles to communicate with us from far away and that the crop circles are messages from them.
Scientists who have studied the crop circles suggested several possibilities. Some scientists say that a downward rush of wind leads to the formation of the circles--the same downward rush of air sometimes causes the airplane to crash. Other scientists say that forces within the earth cause the circles to appear. There is one problem with all these scientific explanations:Crop circles often appear in formations, like the five-dot formation. It is hard to believe that any natural force could form those.
One explanation given by scientists for the crop circles is that they are made by ______.
A、airplane crashes
B．new farming techniques
C．unknown flying objects
D．air movement