老师:您好!

我在做题过程中遇到下面两道题:

1.when considering the riskfactor for a infant riding a car, one is exploring ____ factors.

A. host

B. Agent

C.Enviroment

D. Vector

2.Jack really wanted a bicyckle, but his family couldn't afford one so Jack and his uncle built one from old parts. Jack was around a corner and the bike slipped under a car.Examed by the police reveraled structured problems with the bicycle that caused the accident.These problems are an example of ____ factors

A. host

B. agent

C. enviroment

D. Socioculture

关于这两道题的知识我知之甚少,其中的host, agent, enviroment, vector... 具体代表什么,其定义是什么?尤其第二道题,为何选agent .请老师给予答复,谢谢!