dM (a) Explain why M(t) can only obey the above differential equation if M > 0 (once M drops to 0, it is usual to assume dt =0). Choose the correct answer below. O A. The differential equation becomes undefined at M(1) = 0. B. The differential equation is negative and at M(t) = 0 there is no ethanol left in the body. O C. The differential equation is positive and at M(t) = 0 there is no ethanol left in the body. O D. M(t) = 0 does not exist. (b) If a person's blood alcohol concentration is 2.1 g/liter at midnight, what will his or her blood alcohol concentration be at 7 AM? You may assume that he or she drinks no more alcohol after midnight. His or her blood alcohol concentration will be 0.798 at 7 AM. (Type an integer or decimal rounded to three decimal places as needed.) (c) At what time will their blood alcohol concentration drop to 0 g/liter? Their blood alcohol concentration will drop to O g/liter by : V (Round to the nearest minute as needed.)