In the 1950s, Christian Anfinsen demonstrated the renaturation of the protein ribonuclease (RNase) in vitro. After reduction (to reduce the disulfide links) and the addition of urea (to denature the protein), the protein was in an unfolded state. After removing the urea and the reducing agent, the protein refolded, with greater than 90% activity. If the urea were removed after oxidation occurred, the protein had less than 5% activity. Why would the protein not refold correctly if the urea were removed after the reducing agent was removed? (In other words, what would happen if the urea were removed after oxidation?) Choose the best answer.

a. Urea would participate in weak bonding interactions with RNase, preventing oxidation of Cys.
b. The protein would not fully unfold (denature).
c. Disulfide bonds are not positioned correctly unless weak bonding interactions are present.
d. Contaminants in the RNase preparation would form covalent bonds with the protein, preventing reactivation.