A Hospital Patient Receives Intravenously 15 Units of a Certain $f ( t ) = e ^ { - t / 6 }$

Multiple Choice

A hospital patient receives intravenously 15 units of a certain drug per hour.The drug is eliminated exponentially,so that the fraction that remains in the patients body for t hours is $f ( t ) = e ^ { - t / 6 }$ .If the treatment is continued indefinitely,approximately how many units of the drug will be in the patient's body in the long run? Round to two decimal places,if necessary.

A) 90 units
B) 2.50 units
C) 75 units
D) 21 units