Self-determination, curiosity, challenge, and effort are all in adolescents' motivation.

The following is missing for the question to be complete:

A) extrinsic factors

B) cognitive factors

C) intrinsic factors

D) incentives

  C) Intrinsic factors

Explanation:  Unlike extrinsic motivation factors whose goal and outcome is the acquisition of some tangible goods or other intangible outcome but the outcome is related to some external achievement or success, the intrinsic motivation factor is internal, therefore satisfying some internal aspirations. In this sense, the intrinsic motivation factor encourages actions that are pleasure in themselves, with no apparent tangible material outcome. Intrinsic factors are therefore within the personality, individual, and there is no external reward for such actions, because their desired outcome is personal satisfaction related to the intrinsic aspirations of the personality.

  b.

there are isolated pockets of farming, generally around rivers and oases, which are used for irrigation.

the density would be a smaller number.