rules question- checkraising bet+less than minraise

[2ndGoat]

Player A checks.
Player B bets $100.
Player C raises all-in to $125.

I believe player A can raise here, since the bet by player B re-opened the action for him even though Player C's raise did not.

But what is his minimum raise? I would think to $225 but it's just a shot in the dark.

2nd