21. Which of the following sets of conditions most favors the E1 mechanism?
(A) when the alkyI halide is tertiary and the base is a weak base
(B) when the alkyl halide is tertiary and the base is a strong base
(C) when the alkyl halide is primary or secondary and the base is a weak base
(D) when the alkyl halide is primary or secondary and the base is a strong basc