（ ）forms a natural boundary between Mexico and the USA.
A、The Colorado River
B、The Rio Grande River
C、The American Ruhr
D、The Gulf of California
【正确答案】：B
【题目解析】：The Rio Grande River rises in the southern Rocky Mountains and flows to the Gulf of Mexico, It is about 3,200 kilometers long and forms a natural noundary between Mexico and the United States.美国和墨西哥之间的里奥格兰德河（Rio Grande River）是美墨界河，因为河床经常移位而引起近百年的关于两国边界的争议。
