Now we travel to the north central part of the United States. We are in the state of South Dakota. The land is big and mostly flat with many (36) of corn, wheat and soybeans. But as we travel west, the cropland gives way to wild grasses. A p dry wind blows (37) from the west.
Suddenly, the land becomes (38) and rocky, dry and dusty-no longer green and gold. It is now a light red-brown color. All around are broken (39) forms. There are hills and valleys of all sizes and strange shapes.
These are the Badlands. Hundreds of thousands of years ago the area was (40) , but, then, forces of nature (41) the grass. Water and ice cut into the surface of the earth. They beat at the rocks, (42) them away. The result is one of the world’s (43) sights.
(44) .
The area is a study in extremes. Temperatures in the summer have been as high as forty-six degrees Celsius. (45) .
Life in the Badlands is difficult, but animals do survive. The most well known is the prairie dog. (46) .
，前者比后者少3个中子B．"水玻璃”是硅酸钠的俗称，是一种黏合剂
互为同位素