The correct option is A The resources around an Oasis are limited.
The traditional way of living in the Sahara is nomadic. People adopted a nomadic lifestyle as an oasis can support only a limited number of people for a limited period of time. So when people stay around an oasis for a long time, all the resources that the Oasis can provide eventually gets exhausted. To avoid this, people move in search of new Oasis. This is the traditional way of living in the Sahara.