It was the expected answer, and yet the Doctor couldn't help but laugh. It really wouldn't have sounded all that funny to anyone else, but he'd known the Master for so long that his threats of murder started to become tedious after a while. Plus, he knew better than anyone that the Master hated being laughed at more than almost anything.
"And are you going to? Finally kill me? Uh, four times?" he asked, still laughing. "Provided the whole thirteen thing isn't a rumour. Certainly never bothered you much!"