In selecting PD biomarkers for PK-PD strategies in early research phases, which of the following statements is true?

A. A PD marker ideally is several metabolic steps down from the drug's target of action.
B. Robust animal models can remove the need to test early-phase PK-PD responses in humans.
C. The PD output in animal models should ideally mimic the cell-based assay.
D. PK and PD blood draws should not be performed on the same animals.
E. There are insignificant inter-species differences in PK.