But there is a second reason they can't answer. Jesus has framed the question differently than they would have. To them, the issue was that Jesus was practicing his profession (healing) on the Sabbath which was forbidden. But Jesus puts his healing in terms of good and evil. They could hardly reply that it was forbidden to do good on the Sabbath.
For Reflection: There are often many ways to approach an issue, a question, a problem. Do I seek out the opinions of others on what approach to take? The counsel of someone wiser in the ways of God can be of tremendous benefit.