Which function is the inverse of f(x)=50,000(0.8)^x?

a. f^-1(x)=log0.8(x/20,000)
b. f^-1(x)=log50,000(x/0.8)
c. f^-1(x)=log0.8(50,000/x)
d. f^-1(x)=log0.8(x/50,000)

p. s. there's more points if you answer this same question. i posted it three days ago.

The boiling point of water at an elevation of 0 feet is 212 degrees fahrenheit (°f). for every 1,000 feet of increase in elevation, the boiling point of water decreases by about 2°f. which of the following represents this relationship if b is the boiling point of water at an elevation of e thousand feet? a) e = 2b - 212 b) b = 2e - 212 c) e = -2b + 212 d) b = -2e + 212