key intermediate

例句：

1. 1.The key to solving the problem is to find a solution quickly.
   解决这个难题的关键是迅速找到解决方案。
2. 2. She opened the door with her key and stepped inside.
   她用钥匙打开了门，走了进去。
3. 3. He kept his key hidden in his pocket so no one would know he was coming.
   他把他的关键藏在口袋里，以免有人知道他要来。