American English has so many _________ because the American people have come from all over the world.

A．grammar

B．vocabulary

C．pronunciations

D．dialects

肾上腺素的升压作用可被下列哪类药物所翻转( )

A．M受体阻断药
B．N受体阻断药
C．β受体阻断药
D．α受体阻断药
E．H₁受体阻断药