When levels of CO2 are experimentally increased in a typical ...

[penza]

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.

[strangeaffliction]

I recall doing this last year, it's D, bye bye!