What Explained the Spread of Both Buddhism and Christianity

What explained the spread of both Buddhism and Christianity?

A) Both were militant religions that relied on armies to spread their doctrines to unbelievers.
B) Both rejected any role for women in their religious communities.
C) Both adhered strongly to a single creed and did not adapt to local cultures.
D) Both were religions of monks, whose communities offered protection from the strife that surrounded them.