Manaka $($Pty$)$ Limited incurred the costs as indicated below for the six months ended $31$ December $2023.$ During this period, the total costs fluctuated, but not in direct proportion to the output volume $($units made$).$

Units made Total cost $($R$)$

July $553000$

August $603050$

September $703150$

October $753200$

November $653100$

December $502950$

Applying the high$-$low method, the variable cost per unit is estimated at $..$

a$.$

R$12,50$

b$.$

R $6,00$

c$.$

R$10,00$

d$.$

R $8,00$