In this passage, the author is primarily concerned with

A． advancing an argument against human cloning based on evidence from cloning experiments performed on lower life forms
B． measuring the technical and ethical limitations of the embryonic cloning process in mammals
C． refuting arguments against human cloning through the use of hard scientific evidence
D． illuminating the critical ethical distinctions in process between human cloning and sheep cloning
E． describing how the ethical issues in the quest for human cloning have given rise to new models of embryonic development