Among the following crops, which one is the most important anthropogenic source of both methane and nitrous oxide?

1. A

   Cotton

   Cotton. Not the dual CH4–N2O crop in this list.

2. B

   Rice

   Rice. Flooded paddies: methane from anaerobic soils; nitrous oxide from fertiliser and wet–dry cycles.

3. C

   Sugarcane

   Sugarcane. N2O from fertiliser; not the methane twin of rice.

4. D

   Wheat

   Wheat. N2O possible; not a methane paddy.

Summary. Official key is (b) rice. Standing water makes paddy the main anthropogenic crop source of methane; nitrogen fertiliser and soil processes add nitrous oxide. Cotton, cane and wheat are not that dual source in this set.