I already explained my take on this one, I had hoped convincingly:
"IN business" means not only "hard at work" but "active in the private business world" ("Business " so to speak) as opposed to in the Civil Service ( where people are said to be lazy)---------------
----------------whereas "ON business" defines the purpose of a journey, for instance, but is not the contrary of "Civil Service."
Do you not agree with this distinction?
With only the 2 alternatives I see no other possible justification for the one rather than the other. Not a very good clue anyway in my opinion.