`` RMgX + CO_2 \xrightarrow[\text{ether}]{\text{dry}} Y \xrightarrow{H_3O^+} RCOOH `` What is `` Y `` in the above reaction?
(A) `` R _{3} CO ^{-} Mg ^{+} X ``
(B) `` RCOO ^{-} X ^{+} ``
(C) `` ( RCOO )_{2} Mg ``

(D) `` RCOO ^{-} Mg ^{+} X ``