The money multiplier can also be calculated as ________, where a is the currency drain ratio and b is the desired reserve ratio.
a. ( a + b) ÷ (1 + b)
b. (1 + a) ÷ ( a + b)
c. ( a + b) ÷ (1 + a)
d. a ÷ ( a + b)
e. (1 + b) ÷ ( a + b)