A2
used to refer to something that you have already talked about
Dictionary examples:
You failed the test? This is why we study, my friend. (0.0)
You're always late? This is why we tell you the wrong time! (0.0)
You ate all the cake? This is why we can't have nice things! (0.0)
What's this we've heard about you moving to Scotland? (17.9)
I've had enough of this! (18.5)
This is why you should always lock your bike up. (34.0)