题目

某双星由质量不等的星体S1和S2构成，两星在相互之间的万有引力作用下绕两者连线上某一定点C做匀速圆周运动。由天文观察测得其运动的周期为T，S1到C点的距离为r1，S1和S2的距离为r，已知万有引力常量为G，由此可求得S1和S2的线速度之比 v1：v2＝____________，S2的质量为____________。 答案：；A professor of biology gave a talk to more than 1000 students on elephants living conditions and the dangers _______ them.  A. face          B. facing      C. faced         D. are facing