Sentence Correction

[BTGmoderatorRO]

One student at the school was not yet 15, yet he was already a master in both chess and in bridge.

A) master in both chess and in bridge.

B) master of both chess and in bridge.

C) master of both chess and of bridge.

D) master both chess and bridge.

E) master of both chess and bridge.

OA is E

Can an Expert give me a more revealing method on how Option E is the answer?