Making new field by calling event onchange

I have one drop down box in a form. The value of that box consists of two values, let say "A" and "B". I want situation like following :

For example, when the page is loaded the default value for drop down box is "A".

When I change the value into "B", I want one or more new field appear in the form. Then when I changed the value into "A" again, the field that appeared before, become disappeared.

Is there any reference or code sample to solve this problem ?

Thanks in advance