When that trick did not work, they try another. They try to force him to take a position on the theory the alternative he proposed would alienate either the Pharisees, who believe in resurrection, or the Sadducees, who don't. That is, the Sadducee asking the question doesn't want elucidation or support; he wants to drive a wedge into the crowd of supporters.
The question was based on the requirement in Deuteronomy that if a man died without a heir, then his brother should marry the widow. They asked then if a seven brothers followed their law one after the other, at the resurrection, whose wife would she be?
Jesus responded first by stating that life after death is not like life before it. Resurrection is not merely resuscitation (also see 1 Corinthians 15:35-57).
Then he uses a reference to Exodus to remind them that those who are dead to us are alive to God.
I've used Sharon Ringe's commentary on Luke in this comment and now quote directly:
Jesus' response ends the riddles (20:39-40) but not the opposition. One who speaks so well that he is able to defuse the arguments of the powerful presents a clear and present danger to the public order, and appropriate steps must be taken.