What is A?
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                                                                   Oxymecuration and demercuration of alkenes
This reaction is hydration of alkenes using oxymercuration demercuration method. Oxymercuration demercuration method is one of the best way for the synthesis of alcohol using Markovnikov.'s rule because of the lack of rearrangement of the carbocation. Reagent used is mercuric ion in the presence of dilute sulfuric acid followed by reduction using NaBH₄. During Oxymercuration demercuration mechanism there is an intermediate which has three membered  ring on the pi bond of the alkene and  due to this three membered  ring the reaction has no possibility of undergoing any kind of rearrangement. Now this reaction in according to Markovnikov addition and the product which you are showing here is one 1-methylcyclohexanol  so the reactant can be either of two shown in the picture(methylidenecyclohexane or 1-methylcyclohexene) 
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Oxymecuration and demercuration of alkenes(summary) -Important points to remember -
1. Reaction falls in redox chemistry.
2. Reaction is hydration of alkenes to alcohol.
3. Reaction follows Markownikov's rule
4.No carbocation rearragement due to formation of three membered ring intermediate.