The united states department of homeland security threat advisory system measures threat as severe, high, elevated, guarded, or low. in this system, threat is a variable.

Discrete and ordinal  
These can be seen as factors that are all out because of the level of threat and as such treated as discrete or subjective factors. They can also be regarded as absolute factors can be additionally sorted as either ostensible, ordinal or dichotomous.

The correct answer is a. when they started making tools that will them in cultivating lands, they could raise crops and that made them attached to one place for a longer period of time. after that, they started domesticating animals and building their own permanent settlements that resemble the ones we have today.