An exponential function f is defined such that the value of f(x) decreases by 5% for each unit increase in x. The graph of the function in the xy-plane has a y-intercept of 55. Which of the following could be the equation of this function?

a) f(x)= 55(0.95)^x
b) f(x)= -55(0.95)^x
c) f(x)= -45(0.05)^x
d) f(x)= 45(0.05)^x