Which one of the following statements is not correct?

• A
  The process of settling down of colloidal particles is coagulation.
• B
  The mass in milligrams of lyophilic sol which protects the coagulation of $10 \ mL$ of a gold sol on adding $1 \ mL$ of $10 \% \ NaCl$ solution is its gold number.
• C
  The layer of positive or negative charge acquired by selective adsorption of ions on the surface of a colloidal particle is electrokinetic potential.
• D
  The potential difference between the fixed layer on the colloidal particles and the diffused layer of opposite charge is zeta potential.