题目

His sister left home in 1998, and_______ since.    A. had not been heard of           B. has not been heard of    C. had not heard of               D. has not heard of  答案：B 解析: 略家鸽体温不受环境温度变化的影响，能保持相对稳定，属于恒温动物．相对恒定的体温对动物的生存的意义是（　　） A、增强氧气的运输能力B、使体内产生的热量多C、增强对环境的适应能力D、使体内的新陈代谢加快