How do I generate a table of IDs which have only one attribute each?

I have a table that looks like this

id   attribute
1    a
1    a
2    b
2    a

And I want to collect all of the IDs which have ONLY attribute a. So in the example case:

id
1

My initial thought was to use a where, but that would return:

id
1
1
2

Because 2 also has an "a" attribute in one instance.

P.S. I realize the phrasing of the title is ambiguous; maybe there's a better term than attribute to use in this case?