Salter Corporation uses the FIFO method in its process costing system. The company reported 31,000 equivalent units for materials last month. The company's beginning work in process inventory consisted of 9,000 units, 40% complete with respect to materials. The ending work in process inventory consisted of 7,000 units, 70% complete with respect to materials. The number of units started during the month was:

a. 29,700 units

b. 28,200 units

c. 32,600 units

d. 27,700 units