Refer to the figure.
These results show that

A) ground-nesting warblers beg more than tree-nesting warblers.
B) some ground-nesting warblers, such as the black-and-white and Louisiana waterthrush, beg at lower frequencies than tree-nesting warblers.
C) both tree-nesting and ground-nesting warbler chicks beg at a high sound frequency.
D) tree-nesting warblers beg at a lower sound frequency than ground-nesting warblers.