Darrow, and his board game, needs revision

[Ashish1354]

Which revision best combines ideas?
Darrow invented a board game he called “Monopoly.” Monopoly made him a millionaire.
This did not happen at first.

(b) Darrow invented a board game he called “Monopoly,” which made him a millionaire,
although not at first.

(c) Because Darrow invented a board game he called “Monopoly,” Monopoly
made him a millionaire. This did not happen at first.

I chose c) but correct answer is b)
can someone explain why c) is wrong

[Gajey]

i think that that word can not appear twice like that,there is no sense.
the second use could be replaced by a pronoun,,,,