题目

天然水受到污染后，净化时需要运用科学的方法。 (1)天然水污染的来源有________________________________________(写出一种即可)；常用的净水方法有________________________(写出一种即可)。 (2)漂白粉可用水的杀菌消毒，其有效成分是次氯酸钙[化学式为Ca(ClO)2]。次氯酸钙可发生如下反应：Ca(ClO)2+X+H2O＝CaCO3↓+2HClO,则X的化学式为___________。次氯酸钙中氯元素的化合价是___________________________________。I am sorry that I can not meet you at the railway station. I _______ Beijing by the time you come back from abroad.A. have left B. will have leftC. will leave D. am leaving