SSRS 2005

[Mahain]

Hi all,

i am working with SSRS 2005

i am using multivalue dropdownlist in my report if have three option
in my dropdownlist box for ex "a","b","c"

i have one another parameter(suppose startdate) whom i want to
enable(should me todaydate default) only when user selects option "c".
for all other option it should be disable or blank i am using
following expression

=iif(Parameters!Select.Value="c",Format(Today(),"s hort date"),false)
i also tried
=iif(Parameters!Select.Value="Range",Format(Today( ),"short
date"),nothing)
but in both case the i am not able to make it blank or disable it.

please help me to come out of this issue...........