IMPOSSIBLE means no one can do it.it just simply can’t be done on the other hand,IMPROBABLE means that something is unlikely to happen (so although it can be done,most people probably won’t do it) This task is impossible to complete by today !It seems improbable that the two of you have not met before!(即使天各一方,两个陌生人也有可能碰面,所以不应用Impossible,而用Improbable)
