How does ubiquitylation specifically target proteins for degradation?

• A. Ubiquitin tags proteins, directing them to the proteasome for degradation.
• B. Ubiquitin binds calcium to proteins, marking them for degradation.
• C. Ubiquitin phosphorylates proteins, marking them for degradation.
• D. Ubiquitin cleaves proteins, marking them for degradation.

Answer: (A)