When levels of CO₂ are experimentally increased in a typical grassland community, C₃ plants generally respond with a greater increase in productivity than C₄ plants. This is because

A) C₃ plants are more efficient in their use of CO₂.
B) C₃ plants are able to obtain the same amount of CO₂ by keeping their stomata open for shorter periods of time.
C) C₄ plants don't use CO₂ as their source of carbon.
D) C₃ plants are more limited by CO₂ availability because they lack mechanisms to prevent transpirational water loss.
E) C₃ plants have special adaptations for CO₂ uptake, such as larger stomata.