secret as the grave

例句：

1. 1. The government has kept the details of their secret plan under wraps.
   政府一直保守着他们的机密计划的细节。
2. 2. She kept her secret to herself until she was ready to reveal it.
   她一直保持着自己的秘密，直到准备好透露它。
3. 3.He had a secret that he didn't want anyone else to know.
   他有一个秘密，他不想让其他人知道。