The Accumulated Depreciation's account balance is the sum of
Depreciation expense recorded in past periods.
True/False

I think True

21. Supplies are recorded as assets when purchased. Therefore, the credit to supplies in the adjusting entry is for the amount of supplies:
a. that are in the ending balance
b. purchased
c. used
d. either used or remaining

I think it is B

In the simplest terms, the answer to your first question is correct. However, if an asset is disposed of, then you clear out the accumulated depreciation for that asset.

The answer to your 2nd question is incorrect. Lets think about this. When you buy an asset (supplies) the journal entry would be:
Dr. Supplies
Cr. Cash (or however you paid)

Your answer (purchase supplies) would do the above journal entry. If you Credited supplies, would you be reducing or increasing supplies? In the above entry you are increasing supplies. So, when you credit supplies, you are decreasing them... so... you are accounting for the supplies used. Does that make sense? The correct answer is "C"