Engine choices for NMA

It looks increasingly likely that next Boeing will launch the NMA -  or MoM, or 797, or whatever you want to call it.
One of the most interesting questions is the engine choice(s) that Boeing will make:
First of all, will the customer have an engine choice? Here the questions are:

-   does Boeing want to give the customers a choice
- how large the engine OEM think the market for the new aircraft will be