$1-4.$ Distinguish between $($a$)$ a variable cost, $($b$)$ a fixed cost, and $($c$)$ a mixed cost.

$1-5.$ What effect does an increase in the activity level have on$$

a$.$ Average fixed costs per unit?

b$.$ Variable costs per unit?

c$.$ Total fixed costs?

d$.$ Total variable costs?

$1-8.$Managers often assume a strictly linear relationship between cost and the level of activity. Under what conditions would this be a valid or invalid assumption?