sign compare flip-flop

例句：

1. 1. The signof the new moon is visible in thesky.
   新月的标志在天空中可见。
2. 2.The policeman gave a nod ashe passed by.
   警察点了点头，他经过时。
3. 3. The signoutside the store said "Closed for the day."
   商店外的招牌写着“今天关闭。”