Which is why…

We use this expression to give a reason for something.

You can use this phrase in ………… language.

 Spoken
 Written
 Informal
 Formal

Example 1
A: It’s so hot today!
B: Which is why I’m staying at home.

Example 2
A: What time are we leaving?
B: The traffic’s bad after five, which is why I think we should leave at about 3.