On a weather map, places that have the same temperature are joined with lines called

Isotherms are lines found only in weather maps and the function of these lines are two group areas and regions, that will be having the same temperature. Note, that isotherms are used uniqueley to express temperature, and nothing else.

The image below, shows how isotherms look like.

Hope it helped,

BioTeacher101

the correct answer would be 6 feet.