Would you mind _______ quiet for a moment? I'm trying _______ a form.
A:keeping; filling out
B:to keep; to fill out
C:keeping; to fill out
D:to keep; filling out
每天早上和傍晚都会有很多人在绿树成荫的公园中锻炼身体,他们认为这个时间的空气质量比较好.请结合你所学的知识,就空气中氧气含量而言,你认为一天中比较适宜锻炼身体的时间是( )