On Average, Americans Have Almost Twice as Much Currency as Canadians

Multiple Choice

On average, Americans have almost twice as much currency as Canadians. What is the most plausible reason for this?

A) Canadians trust their banking system a lot more.
B) Americans are a lot richer than Canadians.
C) The American economy is two times bigger than the Canadian economy.
D) A lot of U.S. currency is held by foreigners.