Explain the formation B and C, optically pure different isomers form (A) with little recemisation.

Explanation

(A)
(B) is obtained by `SN^(2)` by the attack of nucleophile `(MeOH)` at less subsituted C, without changing id S-stereoisomer.
(C) is obtained by `SN^(1)` ring opening to give stable `3^(@)C^(o+)`. The nucleophile `(MeOH)` attacks form the backside because frontside attack is blocked is blocked by the `(--CH_(2)OH)` group.