Which box to check? Single vs Multi

We have a plan that was set up as a multiple employer plan in 2018. For 2018 there was only one company in that plan. In 2019 at least two other companies joined/adopted. For the 2018 5500, should we check the single employer box or check the multiple employer box and just have the attachment specify the one company making 100% of the contributions? 

For what it's worth, since it was set up as a multiple employer plan, I would check the multiple employer box and report the one company making 100% of the contributions.  I think avoids any future issues when the next return is filed.