How to make Select Options Mandatory?

Hi Guys,

Can anybnody tell me how to Declare Selection Options for some of them mandatory.

Select-Options : v_PurOrg for EKKO-EKORG default '2000',

v_GRDate for Sy-Datum ,

v_GRPrd for MSEG-GJAHR.

In these i have to make v_PurOrg Manadatory.

Thanks,

Gopi.