populating a textfield dependent on dropdown list and checkbox

Hi Guys,

I need help.
first step: I have a dropdown list if i click in dropdownlist "a" and put in numeric field a number bigger than 99 the, checkbox should be marked and second step: a textfield1 should be visible.

the first step works perfectly but the second doesnt work.
this is my coding for the second step in event : change java script

if ((DropDownList1.rawValue == "a") && (this.rawValue == 1))
{
TextField1.presence = "visible";
}
else
TextField1.presence = "hidden";

does anybody have any idea why?

this is the attached form:

https://acrobat.com/#d=x273XZc4Qd8BjSK-gL9V0w

thanks!!

Diana