c$.$ For shelf life considerations:

Average Daily Demand $=$ Annual demand $/$ Number of days in a year $=720/360=2$ units per day.

Explanation:

Average Daily Supply in EOQ$=30/2=15$ days

$(1)$ For a $20-$day shelf life, you can have up to $40$ units $(20$ days $\backslash$times $2$ units per day$).$ The EOQ of $30$ units is acceptable.

$(2)$ For a $10-$day shelf life, you can have up to $20$ units $(10$ days $\backslash$times $2$ units per day$).$ The EOQ of $30$ units is not acceptable as it exceeds the shelf life capacity.