题目内容：

用方框中所给单词的适当形式填空，使短文完整、通顺。每词限用一次。

able，alien，circle，express，might, mystery，outdoors，purpose，see，valuable

Crop circles(麦田怪圈)are patterns in fields that appear overnight. At first they were just some simple1.，now the patterns have become very complicated(复杂的)．From a plane crop circles can look like a work of art 2.

Since the first crop circles were found in Britain in 1978，it has become a great3.．How these crop circles are made？Who makes them？What is the4. of making them?

Some scientists think that some of the crop circles 5. be man­made. They believe that thanks to modern technology，it is possible to create such complicated patterns in a short time. However，others think that crop circles might be the work of some6.. Because they often appear in areas shortly after there have been UFOs to be 7.．They also think that the circles are so complicated that nobody on earth is 8. to make a true crop circle.

Although different opinions about these crop circles have been 9.，one thing everybody agrees on. That is these crop circles are10. to the local economy(经济)．Thousands of visitors come here to see crop circles every year.