Do I need to have a round trip to China and back or can it be one-way to China?

I may be working in China and renewing every month in HongKong; however, I don't know how long I will be working there. I have a Visa now that will be renewing soon. I am in the USA now. Do I need to show a return flight to the USA from China as part of my application or can it be one-way to PRC.