How did early humans survive their harsh environments?
a.
they lived in caves and rock shelters.
b.
they built walls around their homes for protection.
c.
they built homes out of stone.
d.
they built homes out of wood.

A.  They lived in caves and rock shelters.

Explanation:

The first human beings were nomads, they moved from one place to another looking for areas with hunting and abundant plants. Sometimes they lived in the same place for extended periods of time, while hunting, fishing or wild vegetables were abundant. When food began to be scarce, they marched in search of a new settlement. They lived in caves, in rocky shelters or outdoors.

In the caves, they used to use the entrance of the room as a room, and in the innermost cavities they made their paintings and the supposed rituals. Sometimes, the caves, because of their situation, were privileged places for a group of hunters: they constituted an adequate observatory, from which the passage of the animals could be controlled or sighted when they came to drink.

it would be d). more resources must be obtained to attain that combination of goods and services