Darrow, and his board game, needs revision

English as a Second Language English Vocabulary, Grammar and Idioms
1

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.

© 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

2

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