In which of the following do(es) the elimination step occur by the E1cB mechanism?

A3-hydroxybutanal with dilute $NaOH$ , warm, giving but-2-enal
B $C X_{6} H X_{5} SO X_{2} CH X_{2} CH X_{2} OCH X_{3}$ with $NaOEt$ , giving phenyl vinyl sulfone
C $(CH X_{3}) X_{3} C - Cl$ in boiling ethanol, giving 2-methylpropene
D2-bromopropane with alcoholic $KOH$ , giving propene

Answer: A, B

Dehydration of 3-hydroxybutanal under base and loss of

CH X_{3} O X^{-}

from the sulfone proceed through carbanions stabilised by C=O or

SO X_{2}

(E1cB). Tert-butyl chloride in ethanol undergoes E1, and 2-bromopropane with alcoholic KOH undergoes E2.