What grade are these guitar songs?

Question: What grade are these guitar songs!?

plug in baby - muse
wish you were here - pink floyd
stairway to heaven (intro) - led zeppelin
canon rock (rhythm)

I have an audition for Higher Music (Advanced Level in Scotland) and I need to prepare two contrasting pieces!. For Higher I think it has to be at least grade 5!.!.

also can anyone advice me what scales I should learn!?
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Best Answer - Chosen by Asker:

Guitar songs aren't graded: they're just ranked in terms of difficulty (easy, medium, advanced)!. I believe only piano and marching band music have grades!. Reason being is because these instruments all use books when taking lessons that are of different grades!. With guitar this isn't possible!. (That's why guitar is awesome!) :-D

I'll rate these songs on a scale of 1-5, 1 being really easy:
"Plug in Baby"-3
"Wish You were Here"-5
"Stairway to Heaven intro"-4 1/2
"Canon Rock"-3 1/2

Here'a a link to the five scales you should know:

http://www!.cyberfret!.com/scales/basic/pa!.!.!.Www@QuestionHome@Com