21. The primary advantage C4 plants have over C3 plants is that
(A) C4 plants can produce CO₂ needed for sugar production in the Calvin cycle more efficiently than C3 plants.
(B) C4 plants minimize photorespiration compared with C3 plants.
(C) C4 plants can produce sugars more efficiently than C3 plants under cool, wet conditions.
(D) relative to C3 plants, C4 plants can keep their stomata open more frequently to limit water evaporation.

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