But for water,it --- impossible to live in the earth.A.is B.

ut for ,表虚拟
is 虚拟不用was,而用were
再问： but for的虚拟 不是 would/could/might + do或者have done构成的吗？
再答： 根据意义 如果有would be ,也是可以的 would/could/might have done 表过去情况的虚拟. would/could/might + do 表现在、将来情况的虚拟.