Which of the following statements is incorrect for a bimolecular nucleophilic substitution reaction $(S_{N}2)$?

• A
  It is a second-order reaction.
• B
  In an $S_{N}2$ reaction,the substrate does not undergo heterolytic fission.
• C
  The rate of an $S_{N}2$ reaction does not depend on the concentrations of both the substrate and the nucleophilic reagent.
• D
  $S_{N}2$ reaction occurs in a single step without forming an intermediate.