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19.CAUSE

Some of the natural wonders are known for their beauty. The Meteor Crater is not one of them. It looks like a big hole in the desert. It ______ by a meteorite hitting the earth thousands of years ago.

20.LARGE

The crater is 4,145 feet across, and 570 feet deep. It is the ______ impact crater in the entire world.

21.THINK

When Europeans discovered it in 1871, they ______ it was the top of a volcano.

22.EXPLORE

Since then, scientists ______ the crater for many years, but there are still many secrets and mysteries hiding inside it.