A．It is being raised each year.
B．It remains constant.
C．It will reach sea level in 9,000 years.
D．It is being lowered gradually.

A.It
B.
B．It
C.
C．It
D.
D．It

【参考答案】

D
解析：细节辨别题 文中讲到美国的平均海拔高度在以每9000年一英尺的速度下降。