A．体积
B．质量
C．密度
D．都不是
答案：C
甲、乙、丙三个数的平均数是80，甲、乙两个数的平均数是84，乙、丙的平均数是75．甲数是______，乙数是______，丙数是______．
My father was reading a book ________ I was doing my homework.
A．before
B．after
C．when
D．while
