天文学家能够测定每颗恒星的亮度。An astronomer can determine the brightness of each star.星星的光亮度各不相同.Stars vary in brightness.用18瓦的荧光灯灯泡有同样的亮度,而用电量却减少75%。With an 18-watt fluorescent bulb you get as much light but use 75% less electricity....
I work out regulary to keep fit.我经常努力锻炼身体以保持健康.He runs to keep fit.他跑步为了保持健康.I think the best way to keep fit is to go in for sports.我想保持身体健康的最好办法就是参加体育运动....