The following data have been collected for four different cost items.

Cost Item Cost at $100$ units Cost at $140$ units

W $$8,000$ $$10,560$

X $$5,000$ $ $5,000$

Y $$6,500$ $ $9,100$

Z $$6,700$ $ $8,580$

Which of the following classifications of these cost items by cost behavior is correct?

Cost W Cost X Cost Y Cost Z

A$)$ variable fixed mixed variable

B$)$ mixed fixed variable mixed

C$)$ variable fixed variable variable

D$)$ mixed fixed mixed mixed