Gold numbers of protective colloids A, B, C and D are $0.50$, $0.01$, $0.10$ and $0.005$, respectively. The correct order of their protective powers is:

Protective power is inversely proportional to the Gold number. A smaller Gold number means a smaller amount of colloid is needed for protection, hence it has a higher protective power.Order of Gold numbers: $D(0.005)<B(0.01)<C(0.10)<A(0.50)$.Order of protective power: $A<C<B<D$.