Practical Products uses a process costing system. The following is known about the assembly department for

the month of July:

$51,000$ units were started

$57,000$ units were completed

$6,000$ units were in work in process at July $1$

How many units were in work in process at July $31$st$?$

$12,000$

$0$

$6,000$

$3,000$