SQL & PL/SQL :: Eliminate Donor With Multi Topic Code

Below is an overs implication of what I need to extract from a donor list and am having some difficulty pulling the correct targets. I need to pull a donor who has had only one topic in the past year.

I only want id number 100 and not 120 since 120 has three topics in the past year, I tried using not exists...etc and can't seem to get the donors with the one topic 'HC' that is being requested.

example:

create table Topic(Idnumber number(8),topic varchar2(4));
INSERT INTO Topic(idnumber,topic)
VALUES (100, 'HC')
/
INSERT INTO Topic(idnumber,topic)
VALUES (120, 'HC')
/
[code].......

IDNUMBER TOPI
---------- ----
100 HC
120 IRS
120 PRS
120 HC

SQL> select idnumber from topic where topic in('HC');

IDNUMBER
----------
100
120

SQL> select idnumber from topic where topic in ('HC') and topic not in('IRS','PRS');

IDNUMBER
----------
100
120