with和for当因为讲的区别

下面这句话为什么用with不用for:Thereasonwhyshehadnotstoppedattheredlightwasmostprobablythathereyeshadbecomeweak(with)oldage.分析：with是介词；而for表示“原因”时,通常用作连词,后接从句.例如：Wecan'tgoforitisraining.我们不能走,因为正在下雨.