肺细胞约每周新老更替一次。Lung cells die and are replaced about once a week.昼夜更替.Day and night alternate.从事高等教育的人员新老更替极其缓慢。There has been very little renewal of personnel in higher education....
很少有鸟类会在镜头面前梳理自己的羽毛。Rare birds preen themselves right in front of your camera.燕雀大约在12个星期大小的时候开始换羽毛。Finches start to moult at around twelve weeks of age.拔羽毛必须干净利落。The feather must be removed with a straight, firm pull....