a closely guarded secret的用法和样例：
- Yuxin: Was this also handed down as a closely guarded secret?
- There, the art of making bagels became a closely guarded secret as the Bagel Bakers' Union only admitted sons of its members as apprentices.
- But like almost all Iranian politicians, Ahmadinejad's private life remains a closely guarded secret, and his wife, two sons and daughter are rarely seen in public.
- The Pine Cone Rehabilitation Clinic enjoys a steller reputation as the most successful, luxurious, and discreet rehabilitation clinic in North America. Its client list is a closely guarded secret.
- The experienced roaster, with his knowledge of each bean, artfully combines them to create the desired blend of flavors. The roaster's blending knowledge is usually a closely guarded secret.