What javascript can I use to control the visible items in an html select element?

What javascript can I use to control the visible items in an html select element?

I have an html select element that, based on a value set elsewhere, is supposed to show one of two pairs of value. In one case, it should be "New" and "Assumed", and in the other "Existing" and "Organic Growth"

So currently I'm creating it with all of them, like so:

<tr>
  <td nowrap align="left" valign="top">
    <font color="<%=Session("TextColor")%>" style="font: 8pt arial">Subcategory:&nbsp;</font>  
  </td>
  <td nowrap align="left" valign="top">
    <select name="subcategory" color="<%=Session("TextColor")%>" style="font: 8pt arial" onchange="UpdateFlag=true;">
      <option value="3">Organic Growth
      <option value="2">Existing
      <option value="1">Assumed
      <option value="0">New
    </select>
  </td>
</tr>

But I want to only show two of these values at a time, based on the value of the Boolean IsNewBusiness, which is previously set with VBScript

Assuming that I know the value of IsNewBusiness, how can I specify which pair of items in the html select are visible?