（) can purify the air by absorbing carbon dioxide and releasing oxygen.

al, ethnic, or other groups【C1】______holding them to different standards. The details【C2】______. In some countries, the policy【C3】______only to areas under direct state control, such as public-works contracts or【C4】______to public universities. In others, private firms are also obliged to take【C5】______of the race of their employees, contractors and even owners.【C6】______the effects are strikingly similar around the world. Many of these policies were put in place with the best of intentions: to【C7】______for past injustices and purify their legacy. No one can deny that, 【C8】______, blacks in America have suffered awful wrongs, and continue to suffer【C9】______. Favouring members of these groups seems like a quick and effective way of making society【C10】______. Most of these groups have made great【C11】______. At the same time, the downside of affirmative action has become all too【C12】______. Awarding university places to black students【C13】______lower test scores than whites sounds reasonable,【C14】______the legacy of segregation. But a study found that at some American universities, black applicants who scored 450 points worse than Asians on entrance tests were【C15】______likely to win a place. That is neither fair on Asians, nor a(n) 【C16】______to blacks to study in high school. The book "Mismatch" produces evidence that【C17】______affirmative action reduces the number of blacks who【C18】______as lawyers by placing black students in law schools for which they are【C19】______, causing many to drop out. Had they attended less demanding schools, they might have【C20】______.

【C1】

A．with

B．in

C．at

D．by