#1
And I'm kinda wondering what key I'm in in this arpeggio. Anyone help me please?

e|----------0-----------------0----------------0----------------------------|
B|------------0-----------------0----------------0-----------------7-------|
G|-----8-------8--------8--------8-------8-------8--------8-------8-----|
D|---9---9-------9---9---9--------9---9---9------9----9---9-------9--|
A|-7----------------------------------------------------0-------------------|
E|-------------------7-----------------9------------------------------------|

I'm thinking E something. Could someone give me the exact key I'm in? I'd like to make a song out of this. Thankyouuuuuuuuu
#3
How can you tell that it is in E major? F#, C#, D#, and G# are not present in the riff, which is why I am asking. Please explain, I'd like to learn.
#4
F# = 7th fret B string
C# = 9th fret low E string
D# = 8th fret G string

There isn't a G# which makes it slightly ambiguous, but the E B D# signals a major tonality. also assuming this bar repeats, you have a V-I cadence (B-E)