Once harm ( )to the environment,it takes years to have the s

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Once harm ( )to the environment,it takes years to have the system recovered 括号里填什么,为什么
A.does B.is done C.will be done D.be done

B
once 条件句是do harm to...“对.有害” 的被动式,排除A.
D 的be 形式有误.条件句不用将来时,排除C
一旦环境遭到破坏,数年后都难以恢复
再问：为什么be的形式有误，什么时候用be,什么时候用am/is/are?
再答：一般现在时，当主语是I, He, She, They 等时，如果要用be 动词，便用am, is , are, 注意人称和数

Once harm ( )to the environment,it takes years to have the s Once harm is done to the environment,it takes years to have once harm____(do)to the environment,it takes years to have t 四道英语句型转换,1.What you have done does harm to the environment.W lt is harm to the environment有什么错么? waste gas is _____(harm) to the environment What harm has human beings done to the environment? Once harm___,it takes many years to have thescar cured ,even Once environmental damage ____,it takes many years for the s Takes care of you body.It's the only the place you have to l it's our duty to help ____(protect)the environment It takes me three years to write the book.就three years 提问?怎么