Unlike britain, france, and russia, the united states
a. controlled large areas of china.

b. wanted to keep trade in china open.

c. had no interest in trade with china.

d. broke china into distinct spheres of influence.

China is currently largest goods trading partner with United states of $598 billion during the year 2015. So United states want to keep trade with china.
So according to above explanation,
B. wanted to keep trade in China open, is the correct answer.

North korea has been ruled by dictator since 1948