Suppose the total number of new flu cases recorded in the state of Texas during each day of the month of December was modelled by ( ) = 5 ² + 2 0 where represents the day of the month in December. On what date was the number of new flu cases increasing at a rate of 1 6 0 cases / day ?
[ A ] December 1 0 th
[ B ] December 1 2 th
[ C ] December 1 4 th
[ D ] December 1 6 th
[ E ] December 1 8 th