Mary has many nice dresses. She wears a different one every day        the week.

A．by

B．from

C．at

D．for

（）变化对世界的生态系统和经济社会已经产生并将继续产生重大影响，适应和减缓这种影响已成为当今国际上关注的焦点和国际环境外交斗争的热点。