Mrs. White became a teacher in 1985. She _____ for twenty years by next summer.

A．will teach

B．would have taught

C．has been teaching

D．will have been teaching

下列有关氧化还原反应的叙述中，正确的是

A．一定有氧气参加

B．还原剂本身被还原

C．氧化反应先于还原反应发生

D．一定有电子转移（得失或偏移）