In a fixed exchange rate​ system, how do countries address the problem of currency market pressures that threaten to lower or raise the value of their​ currency? A. If demand​ falls, then countries must increase demand by buying up the excess supply with domestic currency. B. If demand​ rises, countries must fill the excess demand for foreign currency by selling their reserves. C. If demand​ rises, then countries can adjust the value of the exchange rate to the desired level. D. A and B only.