we haven't settleed the question of --- it is necessary for

If 和whether的区别：

if 和whether都可以用语宾语从句中,表达“是否”,但是有两点要注意：A.有介词时只能用Whether,不用if；B.Whether...or not,固定搭配

if 不能引导表语从句,表达“是否”时,表语从句中只能用whether.
eg.The question is whether the enemy is marching towards us.

主语从句中,表达是否时只能用whether引导
eg.Whether he will come is still a question.
上面你那个句子是宾语从句,因为前面有个介词,只能用whether不能用if.