Total demand $($D$)$ is $350$ units, the holding cost per unit $($Ch$)$ is equal to $(0\mathrm{.}6)$ and the ordering cost per order $($Co$)$ is $(11\mathrm{.}5).$ Calculate the total costs for each plan and state which one is more preferable along with the reason why. $[4$ marks$]$

$($c$)$ There is a stark difference in the composition of the total costs of Plans A and B$.$ Explain this difference and why it occurs. Use the breakdown of costs for each plan to help illustrate your answer. $[4$ marks$]$

$($d$)$ Neither plan provides the economic order quantity $($EOQ$).$ Calculate two new plans $($C and D$)$ with order quantities of $150$ and $100$ respectively