对α2受体有选择性激动作用的药物是

A. 15天
B. 30天
C. 45天
D. 60天

A. at 4,000 F, at sea level
B. at 4,000 F, 5,000 feet below sea level
C. at the exact center of the earth at 4,000 F
D. at 4,000 F, 5,000 feet above sea level