Online purchases: The income from annual online sales is approximated through the function:

f(t)=0.075t3+0.025t2+2.45t+2.4 (0t4)

where f(t) is measured in billions of dollars and t is measured in years, with t=0 corresponding to the beginning of 2012.

a. Determine an expression for the reason for changing annual online sales revenue at any time t.

b. How fast did the income from annual sales on the Internet change at the beginning of 2015 ?

c. What was the income from annual sales on the Internet at the beginning of 2015 ?