Request a high efficient query

Have a table T with the following data
T (c1 number(10), c2 varchar2(10))

c1    c2
1      B
1      C
2      A
3      D
4      A
4      A
5      A
5      B
6      C
6      A
6      D
7      E
8      B
8      C
....

Now want to get all C1 with only C2=A (single and duplicates). For the above example,

Want to get the records:
c1
2
4

Since C2 in all of the records are 'A', they are selected..