单选题 A large______of the sunlight never reaches the earth while infra-red heat given off by the earth is allowed to escape freely.
【正确答案】 A
【答案解析】解析：本题考查句意和词组。a large proportion of“一大部分”；in a ratio of“以……的比例”；at a rate of“以……的速度”；a fraction of“一小部分”。