普萘洛尔的降压机制是（）

A.阻断血管平滑肌上β受体

B.阻断血管运动中枢β受体

C.阻断外周突触前膜β受体

D.阻断心肌上β₁受体

E.阻断肾球旁细胞上β₁受体

One of your crew members falls overboard from the starboard side. You should IMMEDIATELY（）.

A.apply left rudder

B.throw the crew member a life preserver

C.begin backing your engines

D.position your vessel to windward and begin recovery