这个实验室的配方是严格保密的。

The formula in this lab is a ___closely-guarded secret.

公司的新产品配方被严密保护着，只有少数高管知道。

The recipe for the company's new product is a ___closely-guarded secret known only to a few top executives.

这个秘密实验室被严密保护，外人无法接近。

The secret lab is a ___closely-guarded facility that outsiders cannot approach.