A Drug Containing with an Activity of 2 $\mu$ Ci Is to Be Injected into a Patient at 11:00

A drug containing with an activity of 2.00 $\mu$ Ci is to be injected into a patient at 11:00 am.A sample is prepared 4.00 hours before the injection (at 7:00 am) .The activity of the drug have at the preparation time (7:00 am) was 3.16 $\mu$ Ci.What is the decay constant of ?

A) 0.165 hr^-1
B) 0.114 hr^-1
C) 0.0793 hr^-1
D) 0.204 hr^-1
E) 0.0961 hr^-1