Is Cobb's reaction honest or sarcastic/angry

In Inception there is the following dialogue between Cobb and Ariadne (forgive me for not being word-accurate):

Ariadne: Why can't you return home?

Cobb: Because everybody thinks I killed her (Mal).

...

Cobb: Thank you.

Ariadne: For what?

Cobb: Not asking if I did it.

I am not completely sure how to interpret Cobb's reaction here. It may be obvious, but while he seems to really thank her for not asking (maybe believing she doesn't care or believes he didn't do it), to me he seems kind of angry or annoyed when saying this. But it may be just me misinterpreting his behaviour here (or the German synchronizer distorting it).

So how is his reaction to be interpreted (while this sounds like an open discussion question, I guess there was a clear intent by the writer how his reaction should come across):

• Did he indeed thank her along the lines of "Thank you for not asking, it's refreshing to see somebody not caring about it and just concentrating on the work."?


• Or was it more like "Thank you for not asking and just trusting me not to have done it."?


• Or was it really a sarcastic thank you and an angry/annoyed reaction, like "Thank you for not asking! You're thinking I did it, don't you?"?

I tend to the more obvious first version, but he just seemed kind of angry to me, which would favor the third version.