once you have cured the water pollution,you will have to take steps to avoid it___again.这里填happens,而不是to happen,happening对吗?

once you have cured the water pollution,you will have to take steps to avoid it___again.这里填happens,而不是to happen,happening对吗? Once you have cured the water pollution problem,you will have to take steps to avoid it___again,多选题：A.to happen; B.happening; C.happens; D.happened,选A和B不对，如果单选，应选哪个？ Once harm___,it takes many years to have thescar cured ,even all his life 为什么填is done? Once have.You'll rest This ____ （cure） your cough if you had taken it. 为什么答案要填could have cured? Once you have, asked for nothing是什么意思 ---- do you have a bath?Once a week. Once you have,do not want nothing中文 Once you have obtained the amount of total millimeters You have to send ( ) a doctor at once?填介词 I want to have you once huan翻译中文 翻译You have only been to school once. I wanna have you once,selfishly翻译中文是什么意思 Not once have you copied your sentence correctly.如何翻译 When I need you,you have once again chosen to leave,since you have already You have not to take the measure once.翻译句子,Once once you understand this rule,you'll have no further difficuty.这里可以用while或者when代替once 为什么`` We have no achievement now.Hope that our time has been once long.You have known.