The natural boundary between Mexico and the US is ________ .
A、the Rio Grande River
B、the Mississippi River
C、the Ohio River
D、the Colorado River
【正确答案】：A
【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。