If the Democrats lose only one Senate seat, which one would flip?

[MT Treasurer]

So let's say Democrats retake the House in 2018 but fall just short of a Senate majority because even though NV and AZ have flipped, Democrats have lost all the other R-held seats (they did well in TX and TN, it just wasn't enough) but also one of their own incumbent Senators. Which seat would be a R pickup in this scenario?

I know we like to talk about MO a lot, but I'm inclined to believe that if 2018 is such a massive wave year that Republicans are basically getting clobbered across the country, it would have to be a state where Trump is still very popular, so I went with WV (probably the one state where Trump campaigning for the R nominee could still help the GOP, maybe even in a D wave year).

I also included ND in the poll because Trump won it by 36 points in 2016 and most pundits consider Heitkamp very vulnerable.