卡氮芥为

A．烷化剂
B．抗代谢药
C．铂络合物
D．植物有效成分
E．抗肿瘤抗生素

Passage Three

As is stated in the passage, learning ______.

A．is generally a complex biological process

B．has nothing to do with organic development

C．is mostly independent of external environmental conditions

D．is directed to changes that can be attributed to training