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....
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....