如何清除select-html标签下的选项值

I am using two drop down(first drop-down category and second drop-down sub category) in a page,both drop-down will be loaded dynamically, in which I will be selecting a value from first drop-down accordingly I have to load value to second drop-down.I has done that,but thing is that it will get done for first time.I am using two drop down(first drop-down catego