特布他林的平喘作用机制为
A.阻断M胆碱受体B.抑制磷酸二酯酶C.激动支气管平滑肌的β2受体D.抑制肥大细胞释放过敏物质E.对抗组胺等过敏介质的作用
参考答案:C
We were just about ready to leave _____ it started to snow.
A.when
B.before
C.after
D.since
损失能量是应用柏努列方程式的前提条件。