Most forms of animal camouflage, e.g., matching background colors, masquerade, etc., were described more than a century ago.

A) Direct experimental evidence supporting their function as antipredator devices has been rather limited, but the topic is now being studied more intensively.
B) Direct experimental studies have proven extremely difficult to design, so evidence for their antipredator function has remained indirect.
C) Large numbers of experimental studies supporting their function as antipredator devices were performed early on, but the homogeneity of the results has caused a recent decline in such research.
D) The number of experimental studies supporting their function as antipredator devices likewise been extensive.