For2For Limited Operates a Process in Which There Is Opening

For2For Limited operates a process in which there is opening and closing work in progress. At the start of May, opening work in progress consisted of 4,000 units which were 75% complete with respect to materials. 36,000 units of finished production were completed during May. At the end of the month, there were 5,000 units of work in progress which were 80% complete with respect to materials. What is the number of equivalent units of material added to the process during the month of May using the first in, first out method of valuing finished production and closing work in progress?

A) 37,000
B) 39,000
C) 40,000
D) 41,000