Many birds are out of ____________ now, because we have done a lot of things to protect __________ birds.

A．dangerous; endangered

B．danger; dangerous

C．endangered; danger

D．danger; endangered

三甲胺是（）在酶或微生物作用下还原而产生的。

A、氨

B、二甲胺

C、三甲胺

D、氧化三甲胺