The concentration of a medication M in mg/L present in the bloodstream decreases over time such that the concentration of medication is proportional to the amount of medication t hours after the first dose. The concentration of medication 30 minutes after the first dose is 50 mg/L and was decreasing at a rate of 33 mg/L per hour. Which differential equation represents this relationship?
1) dM/dt = -33t
2) dM/dt = -33
3) dM/dt = 33t
4) dM/dt = 33