Of the following characteristics, which is unlikely to be associated with the process of facilitated diffusion of drugs?

A、The transport mechanism becomes saturated at high drug concentrations;
B、The process is selective for certain ionic or structural configurations of the drug;
C、If two compounds are transported by the same mechanism,one will competitively inhibit the transport of the other;
D、The drug crosses the membrane against a concentration gradient and the process requires cellar metabolisim