What is desertification? how are nomads and droughts related to this process? (site 2)

Desertification is the formation of desert out of a cultivable land and both nomads and droughts escalates the process of desertification.

Explanation:

Desertification is the conversion of cultivable fertile land into non cultivable barren piece of land.  

Nomads are people who do not settle at one place. They keep on wandering. Thus, at all locations where they settle, they do cultivate the nearby land after clearing the dense plantation over it. Now while leaving from a particular place they create a land parcel that becomes barren and dry after a certain period of time and lastly convert into desert.

Droughts make a land dry and non-cultivable. A dry land eventually converts into a desert.  

b tumor cells have a cell wall instead of a membrane

explanation:

tumor cells different from normal cells because tumor cells have a cell wall instead of a membrane.