The old man looked at him straight in the eyes and said,“I l

这里用which,是定语从句,which 指代整个句子,它告诉我,这是没用的.yours 意思是你的眼睛,但当我看到你眼睛,我看到了友善.
再问： which told me it would be useless,是什么意思
再答： which 指代I looked into their eyes, I found they didn’t care 这件事，我看到他们的眼神，发现他们一点都不在乎，这件事告诉我，那方法是没用的