reporting (variable selections)

dear friends,

i got a report with circle and subcircle information. i created a characteristic variables, ZCIRCLE for circle and ZSUBCIRCLE for subcircle. when i execute the query the report will ask for circle and subcircle values. my requirement is when i give some circle value the subcircles which comes under that particular circle should exist, sothat i can select required subcircle from the available subcircles. but what i am getting is when i select a particular circle, all the subcircles which does not belong to the circle which i selected is also displaying. any customer exit required. if so please suggest me .

thanks and regards,

hari