The correct answer is - B -Grass is easier for herbivores to digest.
The grassland ecosystems were revolutionary for the herbivores when they started to form, and big portion of them became grassland dwellers where they occupied the niche of primary consumers, because the forest ecosystems where not offering the same amount of advantages for feeding. Compering grasses and trees, the grass has more nutritional value, and it is also much easier for the herbivores to digest the grass, than to try and digest tree branches, thus the logical option for them was to move into the grasslands.
The grassland ecosystems are relatively new on the scene, and they only emerged around 20 million years ago in South America. Since they provided lots of food, and it was high nutrition, easy to digest food, the herbivores quickly moved in, thus increasing the number of primary consumers in this ecosystem.
