The Assembly Department of GigaGo, Inc., manufacturer of computers, had $1,000$ units of beginning inventory in September, and $6,000$ units were transferred to it b the Production Department. The Assembly Department completed $3,000$ units during the month and transferred them to the Packaging Department. Calculate the to number of units accounted for by the Assembly Department if it had $4,000$ units in ending inventory. The weighted$-$average method is used.

A$.1,000$ units

B$.6,000$ units

C$.4,000$ units

D$.7,000$ units