Which of the Following Statements Is LEAST-Likely True

Which of the following statements is LEAST-Likely true?

A) NO_X emissions are highest at a point near stoichiometric.
B) NO_X emissions are more when the mixture is leaner than 16:1.
C) The catalytic converter is more efficient at controlling HC and CO when the air-fuel mixture is stoichiometric.
D) A rich mixture will cool combustion and has less O₂, so NO_X emissions are lower.