What was the original goal of Dr. Zhiyuan Gong and his colleagues when they inserted the green fluorescent protein (GFP) gene into a zebrafish embryo?

A. To create a new species of fish
B. To create a new color variant of zebrafish for the aquarium trade
C. To study the behavior of zebrafish in different lighting conditions
D. To develop a fish that could detect pollution by selectively fluorescing in the presence of environmental toxins
Answer: 