"Well. That's a tough question, actually," The Doctor started, thinking back to the various times and places he'd been lately. "Oh, before showing up here I was on Barcelona." He paused, "The planet, not the city."
And then he chuckled a bit when she stated where she had been last. "Mars, hmm? Oh, Donna would love you. She was always insisting that I was a Martian even though I'm clearly not." Ah, Donna. Another of his old companions that he wouldn't be able to see again. At least this time he was staying away for her health and not because he had no other choice.