During the accounting period, 20,000 units were completed and transferred to finished goods. The beginning goods in process were 4,000, which were 60% complete as labour and overhead. 1,000 units remain in process and are 80% completed. The number of units started during the accounting period was:

a. 17,000 units

b. 20,000 units

c. 18,500 units

d. 21,000 units