I am stuck on these three questions please help!

1) Variable costs with the relevant range for a firm are assumed:

a) Not to vary in total

b) To be nonlinear

c) Not to vary per unit

d) To be curvilinear

2) Manufacturing cost include all of the following EXCEPT:

a) cost incurred inside the factory

b) both variable and fixed costs

c) both direct and indirect costs

d) both direct and period costs

3) Within the relevant range, variable cost

a) is not affected, in total, by changes in activity from period to period

b) remains constand on a per unit basis

c) remains the same in total as production increases

d) increases on a per unit basis as the number of units produced increases