Tom your answers did help. But I donot understand why after I unsoldered the chrome trap and started to apply heat to the copper pipe to unsweat it, did the other side of the 90 become unsoldered ? Is there a different way of unsweating street elbows versus cast copper elbows ? Maybe the copper pipe is not meant to be unsweated and I might have to buy another cast copper elbow. Thank you for your patience.

You do not need a cast fitting that takes so much heat to unsweat a joint that it loosens another joint. Get a regular DWV copper ell and a short piece of copper, sweat on a male adapter and convert to PVC from there. Cheers, Tom