地球自转的周期是（   ）

已知函数f（x）=|x﹣a|+|x+5﹣a|

    It's Sunday morning. The students of Class 3 are giving their classroom a good cleaning. Miss Huang, their teacher, is working with them. The children are busy. Some are carrying water, some are cleaning the windows, others are sweeping the floor. Zhang Hua is putting up a map on the wall. It is a map of China. Wang Fei and Wei Qing are mending some broken chairs. The children are listening to the radio while they are working. The classroom looks nice and bright after the cleaning. The children are very happy. They go home for lunch at noon.