Light limits photosynthesis where rate rises linearly with intensity

Steps:

• Recall photosynthesis rate vs. light intensity graph: initial linear rise (light-limited), then plateau (saturated).
• Identify limiting sections: where slope is positive and steep, rate directly proportional to light.
• Label graph sections: P (initial rise), Q (continued rise), R (plateau), S (secondary rise or extension).
• Select sections with linear increase: Q and S match this pattern.

Why B is correct:

• In Q and S, rate increases proportionally with light (per light response curve), so light is limiting factor by definition.

Why the others are wrong:

• A: P is limiting but R is plateau (CO2 or enzymes limit).
• C: Both R (plateau) and Q (limiting) mismatch.
• D: S is limiting but P may be transitional, not both fully limiting.

Final answer: B