The following data is available for an item of JNC Inc. for the month of March:

$\backslash$table$[[$March $1,$Inventory,$15$ units at $$10$ each$],[15,$Purchase,$30$ units at $$18$ each$],[31,$Purchase,$24$ units at $$15$ each$],[,$Sale,$30$ units$]]$

Using the first$-$in$,$ first$-$out method, what is JNC Inc.$\text{'}$s cost of ending inventory for March?

a$.$ $$510$

b$.$ $$630$

c$.$ $$360$

d$.$ $$420$