Given a text T, and several options, according to the question posed in the text T to choose the most appropriate option from the options as the answer, please directly output the most appropriate option, do not need to output the explanation.

Which of the following best describes a fundamental assumption when monetary policy is used to influence the economy?
Option A:Financial markets are efficient.
Option B:Money is not neutral in the short run.
Option C:Official rates do not affect exchange rates.

Answer: B