Correct Answer: Water limits and fire

Solution : The correct answer is water limits and fire.

Grasslands are formed in zone of transition between forest ecosystem and desert ecosystem. They are also considered as the region of ecotone. Grasslands are usually created when there is lack of water to sustain a forest and because it has vegetation in the form of dry grasses, they are more prone to forest fires.

Because of the above two mentioned reasons grasslands do not become forests as a part of ecological succession.