If Gladys demonstrates plowing immediately before Frank demonstrates threshing, which one of the following must be true?

I understand the part of answer A in that frank must go second as the only possible order of people would be LFLGFG, but I don't understand why Frank must do harvesting second and not as the fifth task. From what I can see the order of tasks with the order of people I listed could be STMPHT respectively and not break any rules, so I don't understand how answer A is a MUST be true and not a could be true.