Given that $Q=$ order quantity, $D=$ annual demand, $C=$ unit cost of the item, $C_0=$ cost per order placed, $i=$ inventory carrying charge per unit, which of the following represents the holding cost per unit?

$\frac{D}{Q}$

$iC$

$(\frac{D}{Q})C_0$

$iC\frac{Q}{2}$