Every way the best bid can change
Forget the mid for a moment and focus on a single touch: the best bid.
Enumerate every way the best bid can move up or down. For each, say whether a trade is required.
Show a hint
A best bid can rise or fall. Rising needs new buying interest; falling needs the current best to disappear. Which of those involves an execution?
Your answer
This one is open-ended. Work it through, then check your reasoning against the full solution.