Try something similar to this: You can also pass variable and add if-else in javascript function to make visible true/false
<select onSelectionChange="javascript:ChangeText()>
functionChangeText()
{
var id = document.getElementbyName('txtArea');
id.Visible = false;
}
<select onSelectionChange="javascr
functionChangeText()
{
var id = document.getElementbyName(
id.Visible = false;
}