In the 1950s, Christian Anfinsen demonstrated the renaturation of the protein ribonuclease (RNase) in vitro. After reduction and the addition of urea, the protein was in an unfolded state. After removing the urea and then the reducing agent, the protein oxidized and refolded, with greater than 90 % activity. If reducing agent removal occurs before removing the urea, the protein showed 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.