Present Perfect Continuous and Future Simple - quiz contemplation

Question

I bumped into that quiz, and I'm quite confused by these two questions' answers. I would appreciate any clarifications.

A) Jane talks on the phone. Bob has been talking on the phone for an hour. Mary is talking on the phone. Who is not necessarily on the phone now?

Answer: Jane

For my understanding, there is a chance that also Bob is not necessarily on the phone, due to the use of the Present Perfect Continuous.

One of the uses (source) of the tense is for Finished actions - Actions which have recently stopped (though the whole action can be unfinished) and have a result, which we can often see, hear, or feel, in the present. We don't use a time word here. Eg. "I'm so tired, I've been studying."

Next question:

B) I'm going to make dinner for Frank. I'm making dinner for Judy. I'll make dinner for Mary. I make dinner for Ted. I will be making dinner for Tony.

Who are you offering to make dinner for?

Answer: Mary

Is it absolutely wrong to say that I'm offering to make dinner to Frank? My friend told me that it sounds natural for him in the given context. I do get that the question is about the differences between "I will" and "Going to"

Answer 1

As your source says, when we use the present perfect continuous to refer to states that have recently finished we don't use a time word (or phrase). "Bob has been talking on the phone for an hour" has a time phrase, so I'd interpret it to mean "the state continues to the present"

The question about dinner, "I'll make dinner" can be interpreted as an offer, and is more likely to be an offer than "I'm going to make dinner". This is not absolute, and you might imply an offer with "going to". But in general, your grammar book is correct. The "will" future is more likely to be used in a conditional context, which is why it is more likely to be used for making an offer. The condition is omitted or implied:

I'll make dinner for Mary (if you go shopping...)/(if you don't want to)/(etc)