What Have Researchers Studying the "Muscular Male Ideal" Found Out

What have researchers studying the "muscular male ideal" found out about GI Joe dolls?

A) GI Joe dolls have become taller over the last forty years.
B) GI Joe dolls have gotten an increasingly smaller waist over the last forty years.
C) GI Joe dolls have had increasingly larger biceps over the last forty years.
D) GI Joe dolls have not changed over the last forty years.