on guard

警惕，防范

on account of, on an average, on an even keel, on and off, on average, on balance, on behalf of, on board, on business, on condition that, on credit, on cue, on diet, on display, on duty, on earth, on end, on file, on fire, on foot, on hand, on instalments, on instinct, on no account, on occasion, on one's guard, on one's head, on one's honor, on one's honour, on one's own, on one's way to..., on order that, on principle, on purpose, on relief, on sale, on schedule, on second thoughts, on the alert, on the analogy of,