They were watching closely to find fault to trap, criticize and to make judgement against Him.  Jesus was asking to see if they were going to continue to require no healings be performed the Sabbath.  He recently rebuked them after healing a woman who was bent over for 18 years from a spirit of infirmity on the Sabbath in the synagogues. Jesus declared the sick being healed by God on the Sabbath in the synagogue would glorify God so how asinine was this rule.

The Pharisees were silent because so many people were happy after Jesus had revealed healing on the Sabbath in the Synagogue would glorify God.  They were afraid they would be the point of anger of the people after Jesus set them free from the spirit of the Sabbath.  Silently in their hearts they wanted Jesus to perform this healing on the Sabbath in order to trap Him so that they would have a reason to destroy Him.

Again, with the Spirit of the Lord and the Wisdom of God, Jesus makes the point of the foolishness of the rules and heavy burdens the establishment placed on people.  He points out that these political hypocrites would do good deeds on the Sabbath if it involved them and the things they owed.

They could not argue with Jesus because they themselves could not keep the rules of the Sabbath based on their own rules.