The Master outlines the scenario in detail, it's clearly one he's thought about a lot.
The Doctor stares out of the window, following drops of rain against the glass. “Don't be ridiculous,” she says.
“You would have chosen Bill over Nardole,” the Master continues, “that was never even a question, but these three... I can't tell which is your favourite.” He hums to himself. “You'd let Graham die, because he's older. The other two have more to live for.”
“Don't be stupid,” the Doctor mutters.
“Would you choose Yaz? You always did prefer girls. But Ryan has a certain clumsy charm to him. But definitely one of those two.”
He really wants to know, she realises. He offers her an impossible situation where she can only save one of her friends, and he demands that tells him who she'd choose. It's twisted and heartless, but that's him all over.
“It isn't real,” she protests.
“But if it was.” He insists, he will always insist.
The Doctor presses her fingers against the glass, watches her nails go white. “Isn't it obvious?”
“Not to me, it isn't.” He repeats himself. “Who do you save?”
Finally she turns her eyes towards his. “You.”