I think the correct answer is A
The clairvoyant principal always seemed to know when trouble was afoot.
Both are a number (6) multiplied by a variable (z).
However, 6z is the correct way to write this.
We usually write the number first, that's just the way it's done.
It sort of makes sense because 6z is literally equal to 6 z's.
There are 13 books in the series
