Which of the following statements BEST describes zero-day exploits?

Which of the following statements BEST describes zero-day exploits?

A. When a zero-day exploit is discovered, the system cannot be protected by any means. B. Zero-day exploits have their own scoring category in CVSS. C. A zero-day exploit is initially undetectable, and no patch for it exists. D. Discovering zero-day exploits is always performed via bug bounty programs.