facultative

形容词 facultative:

1. of or relating to the mental faculties
2. able to exist under more than one set of conditions
3. granting a privilege or permission or power to do or not do something
4. not compulsory

1. The last questions in the examination were facultative.
   最後的那些考题答兴不答可听自便。

fac.ul.ta.tive

adj.（形容词）
Of or relating to a mental faculty.
能力或关于能力的

Capable of occurring or not occurring; contingent.
偶发的：能发生的或不能发生的；或许发生的
Not required or compulsory; optional.
任意的：不被要求或强制的；可选择的
Granting permission or authority.
许可的或授权的
Biology Capable of functioning under varying environmental conditions. Used of certain organisms, such as bacteria that can live with or without oxygen.
【生物学】 兼性寄生的：能在不同环境条件下活动的。用于某些生物，如细菌在有氧或无氧条件下都能生存

【引伸】
fac.ulta.tively
adv.（副词）