Parameter Field - please help

Hello

I am using Crystal Report ver 10. I have three parameter fileds define in my report;

1) Crew (string - values are predefined in history table. They can be more then one)

2) Employee Last Name (string - values are predefined in employee table. They can be more then one)

3) Date Range

I would like to make these parameter fields NOT mandatory. Currently report don't run until I have select or enter data in each of these fields. Is there a way that these fields can become optional and filter data user provided. Any help or suggestion would be greatly appreciated.

Thanks

Syed