Answer:
C. Changes to opportunities are causing cross-object workflow field updates to be made on the account
D. Changes to opportunities are causing roll-up summary fields to update on the account
Step-by-step explanation:
The maximum trigger depth exceeded error would occur when there is a recursion in the trigger. This recursion can be caused by a lot of factors which may include; repeated executing the same code over and over again, an infinite loop resulting in governor limit, cross-object workflow field updates on accounts, and update of roll-up summary fields.