VB Programming

Hey, I've gotten the excel into a program where a student inputs the courses they want to choose for school and excel checks if those courses are there. But, i was wondering how, if you typed 8 courses in 8 text boxes, you would program it to choose the suitable time periods those courses can be in, and just for a check print those multiple time periods in seperate text boxes.

for example
If the first course u pick is only available for periods A,B,C,D,E
If the second course u pick is only available for periods C,E,G,H
etc. How would u get all the combinations for that.