Keyboard accelerators are configurable. You can find out which keyboard accelerators are bound to a command in menu Tools → Options on the Environment → Keyboard page.
These commands are named Edit.CommentSelection and Edit.UncommentSelection.
(With my settings, these are bound to Ctrl + K, Ctrl + C and Ctrl + K, Ctrl + U. I would guess that these are the defaults, at least in the C++ defaults, but I don’t know for sure. The best way to find out is to check your settings.)