tree diagram - student's timetable

Im having trouble drawing a tree for this question. I get 24 outcomes, but its only suppose to be 18.

A grade 9 student may build a timetable by selecting one course for each period, with no duplication of courses. Period 1 must be science, geo, or physical edu. Period 2 must be art, music , French, or business. Period 3 and 4 must be mathematics or English.
Construct a tree diagram to illustrate the choices for a student's timetable.

Here's what I did.

Thanks

yep, vertical lines

but the answer is suppose to be 18 =/ or maybe its wrong..I dunno