Hi, do you mind explaining why first name would be a primary key? There can be many people with the same name.
I thought a user_id would work for all 3 as it is likely to be just a sequential number that can't clash and it won't cause issues if someone wishes to change email/username/etc...
Edit:nvm didn't realise this post was a year old. Hope this helps someone anyway.
Hey, I'm in year 8 so haven't done the course, but know a bit about networking, I think I know why it can't be A.
It can't be a because if an even number of bits change, it still checks out.
Eg: say the message...