Indicate which of the following is correct
A. Increasing the concentration of nucleophile favors an `SN^(1)` reaction over `SN^(2)` reaction
B. ethyl iodide is more reaction than ethyl choride in an `SN^(2)` reaction
C. In an `SN^(2)` reaction, a greater yield of the product with retained configuration is obtained
D. An `SN^(2)` reaction is two step reaction
