Willow Corp uses a FIFO process costing system in which direct materials are added at the beginning of the process and conversion costs are incurred uniformly throughout the process. Beginning inventory for January consisted of 1,000 units that were 40% completed with respect to conversion costs. 10,000 units were started into the process during January. On January 21, the inventory consisted of 500 units. Equivalent units for conversion costs were 10,450. What percentage complete was the ending inventory on January 31 with respect to conversion costs?
a. 70%
b. 80%
c. 90%
d. 100%