LINGUA LATINA HELP

I’m having a little bit of trouble in chapter 9 in Lingva Latina. What would go in the blank for _ arboribus umbra est. ?? I can’t think of a reasonable answer. (pensum B, 3rd line from the bottom)

The correct sentence would be:

Sub arboribus umbra est.

Because arboribus is ablative and needs an ablative preposition — and since it is saying the shadow/shade is ____ the tree. So it is under, which is sub.