Fixed-Rate Relationships Among Currencies Could Not Stay Fixed, According to Obstfeld

Fixed-rate relationships among currencies could not stay fixed, according to Obstfeld and Rogoff, because:

A) the volume of global transactions started to exceed most countries' foreign exchange reserves, so governments couldn't intervene to sustain the value of their currency.
B) the complexity of international trade demanded return of the gold standard.
C) Walmart and other leading firms argued successfully at the Federal Reserve that fixed rates were too costly to maintain.
D) the EU had decided to float the euro.