______ forms a natural boundary between Mexico and the US
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 boundary between Mexico and the United States.参见教材P178。
