Updateing A Field Using A TRIGGER

Hi

I have a table called cur and i want one of the fields to contain a value of two other fields multiplied together

Table 'cur' Fields ID, A, B, C
field C must = A*B

I have this so far and it works but updates ALL of the fields C rows

______________________________________________
ALTER TRIGGER TG_cur
ON cur
AFTER INSERT
AS
BEGIN
Update cur
SET C = (i.A*i.B)
from inserted i
_______________________________________________

RESULT:
ID|A|B|C
1 - 3 - 4 -20
2 - 6 - 6 -20
3 - 8 - 9 -20
4 - 2 - 3 -20
5 - 4 - 5 -20


I think need a where statement at the bottom but the followings does not work
where ID = i.ID

REQUIRED RESULT
ID|A|B|C
1 - 3 - 4 -12
2 - 6 - 6 -36
3 - 8 - 9 -76
4 - 2 - 3 -6
5 - 4 - 5 -20
any help would be great

many thanks

Bil