Which observations would support an E1cB(reversible) mechanism over a concerted E2 mechanism for a base-induced elimination?

1. AUnreacted substrate recovered from $\mathrm{D}{}_2\mathrm{O}/\mathrm{OD}{}^{-}$ has incorporated deuterium at the beta carbon.
2. BThe rate increases when an electron-withdrawing group that stabilises a carbanion is placed on the beta carbon.
3. CPoor leaving groups such as $\mathrm{OPh}{}^{-}$ are expelled readily.
4. DThe reaction needs an anti-periplanar arrangement of the beta-H and the leaving group.