Good question. I learned from my first kumu that "I" and "ma" do have some specific uses, but are essentially interchangeable. This does not seem to be the case on DL, but I've yet to find the key to figuring out the DL-preferred specifics.
Ma and i are interchangeable in this case
For this prompt, if you interchange them, it is marked incorrect.
I believe that either should work... ma seems to flow better when speaking. If anyone has a secret grammar resource for this stuff, please enlighten us.