although the old man is in his sixties,-----he walks every fast.A but B so Cyet Dhowever

选C
表示“虽然”时,根据英语的习惯,在后面不能使用连词 but.
但可以与一些副词连用,如有时 although 可与 yet,still,nevertheless 等副词(或该词在句子中起副词作用时）连用.
however是连词.
句子前半句和后半句是转折的,而so是表示因果的,不符合题意.
如：他虽然年纪很大了,但还很强壮.
Although he is very old,(yet) he is quite strong.