In Intermittent IV Heparin Therapy,the Blood for the Activated Partial $\frac{1}{2}$

In intermittent IV heparin therapy,the blood for the activated partial thromboplastin time (aPTT) should be drawn at which time?

A) 1 hour before the next dose
B) 2 hours after the last dose
C) 30 minutes before the next dose
D) 2 $\frac{1}{2}$ hours before the next dose