Interrogative clause

Hi,

A: Why do adults fight over who's going to pay the bill in restaurants?
B: I would have them fight against each other over who was going to pay me more.
C: As for what should be done next, I think your own proposal is best.
D: They are wrangling over who should be secretary.
E: It depends on how much time we have left.
F: They can't agree (about/on) who is the best person for the job.

These sentences have it in common that the embedded interrogatives clauses follow the prepositions. But the way they are embedded seems to vary.

As for "D" and "E", it is not the preposition itself that made the interrogative possible but the preposition in combination with the verb. For example, you can't have an interrogative after the phrase "rely on", which shows that the preposition "on" is not the sole factor in determining the admission of an interrogative clause.

As for "A", "B", and "C", I think the preposition is the decisive factor since you don't necessarily have to have a specific verb to allow an interrogative clause after the preposition.

I don't know how to deal with a case like "F". Unlike the other examples, the preposition is optional in this case. It is sort of like a hybrid type.

I'd like to hear some opinions on this. Any opinions would be appreciated.