What's the 'T' in auto completion as I type 't', and how to get rid of it?

[DragoonKiller]

This is a newly created untitled file, without any project opened. And here's a 'T' on the top of auto-completion. Each time I want to type identifier 't' it comes out and take the first place... Which is really annoying..

Disabling C# extension will remove all auto completion including the 'T'.