Clearing text box, Dropdown and Check boxes using java script.

Hi,
Here i am trying to Clear text box, Dropdown and Check boxes using java script. Here is the js function followed by html code. For some wired reason few of the drop down list are getting cleared and few are not. None of the text boxes and Check boxes are getting cleared. Any thing that is missing? Please suggest
<script>         function clear() {             document.getElementById("<%= ddlInterviewJobOpening.ClientID %>").option.length = 0; // Not Cleared             document.getElementById("<%= ddlInterviewClient.ClientID %>").option.length = 0;                         document.getElementById("<%= ddlInterviewer.ClientID %&g ...