Refer to the Figure

Refer to the figure.
The figure documents the effects of the loss of bird pollinators on reproductive success in a New Zealand shrub. In this study, bagged flowers allow only self-pollination, open flowers allow bird pollination, and humans carry out hand pollination. Reproductive success occurs when flowers produce seeds. This study suggests that pollination by birds

A) is essential for reproductive success.
B) strongly increases reproductive success.
C) strongly decreases reproductive success.
D) has little or no effect on reproductive success.