A patient has a drug concentration C(0)=10mg/L in the bloodstream at t=0 hours. For 0≤t<2 hours, the concentration satisfies dtdC=0.5C. After t=2 hours the drug metabolizes so that dtdC=−0.25C. At what time t>2 (in hours) does the concentration return to 10mg/L?
a) 4
b) 10
c) 6
d) 8