暗号
7.A smile creates happiness in the home, fosters good will in business, and is the countersign of friendship.
8.They waited for George to give them the sigh sign, and then they slipped silently out the back door.
9.A prearranged reply to a challenge, as from a guard or sentry; a password.
10.A secret sign or signal to be given to a sentry in order to obtain passage; a password.


