They said therefore to him, "What then do you do for a sign, that we may see, and believe you? What work do you do?
What then ... for a sign ...  This demand of a sign was characteristic of that people.  The Pharisees demanded a "sign from heaven"  (Matthew 16:1; 12:38), no doubt meaning some spectacular wonder of their own choosing; but Jesus rejected such vain and carnal demands, resting the final proof of his Godhead upon "the sign of the prophet Jonah," that is, the death, burial and resurrection from the dead.  Mark stated of another occasion that Jesus "marveled at their unbelief" (Mark 6:6).  Surely Jesus must have marveled here also.