The total number of electric cars that are sold each year has been growing exponentially. In the initial year of major production (year zero), 50,000 electric cars were sold world wide. Four years later, the number of cars sold in a year had increased to 500,000. Find the exponential growth constant k and write down an exponential function A(t) that models the number of electric cars sold in year t of major production. According to this model, how many cars will be sold in year ten? How many years (after the initial year of major production) will pass before the number of electric cars sold in a year reaches 100,000,000?