This question ate my brain too, but there is a very simple rule to this, RC (in most cases) won't jump over the verb. So, 'that' can't (again not a rule, but in most cases) jump over is and hence, can only modify communities.
Try the same logic in this official question:
https://gmatclub.com/forum/a-higher-int ... 83528.html
Try the same logic in this official question:
https://gmatclub.com/forum/a-higher-int ... 83528.html






