MEDICAL RESEARCH AT THE CELLULAR LEVEL, ____ RESEARCH ON THE IMMUNE SYSTEM, HAS BEEN MADE POSSIBLE THROUGH TWENTIETH-CENTURY ADVANCES IN TECHNIQUES OF GENETIC RESEARCH

28. Medical research at the cellular level, ____ research on the immune system, has been made possible through

twentieth-century advances in techniques of genetic research.

A. which

B. Whereas

C. such as

D. is also