Canada landing on AP question. 485 pending

I am 485 pending on EAD. I don't have any visa any more. I plan to go to canada for landing and come back on AP. My question is:

How do I answer when the US custom asks me about the purpose of my trip to canada? If I tell them my purpose is landing, will the US side conside me of abandon my US Green Card application? Will I have problem getting back?

Thanks a lot.