宾语从句I don't know _ to visit the old man.A whether B if

A
疑问代词whom、what、which、连接词whether（不能用if）、疑问副词when、where、how常常与动词不定式连用,构成一种特殊的动词不定式短语,可以作主语、直接宾语、表语、介系词的宾语等.如：
How to fight pollution is a problem.
It remains a problem how to fight pollution.
I don't know whether to refuse it or not.
The problem was how to cross the river.
This is a book on how to speak with a pure American accent.
希望我能帮助你解疑释惑.