The full-length coding sequence of an eukaryotic gene was expressed in bacteria and the protein was purified. However, in the functionalassay, no activity was detected for the purified protein. The reason could be

• A. the host bacteria produced an enzyme that inhibited the activity of the expressed eukaryotic protein
• B. the purified protein was contaminated with bacteria
• C. the host bacteria did not produce the essential co-factors
• D. no post-translational modification on the protein expressed in bacteria