The following lots of Commodity $Z$ were available for sale during the year.

Beginning inventory

First purchase

Second purchase

Third purchase

$10$ units at $$30$

$25$ units at $$32$

$30$ units at $$34$

$10$ units at $$35$

The firm uses the periodic inventory system, and there are $20$ units of the commodity on hand at the end of the year.

What is the ending inventory balance of Commodity $Z$ using the weighted average cost method?

a$.$ $$690$

b$.$ $$659$

c$.$ $$655$

d$.$ $$620$