Appear/Dissapear

can anyone plz tell me how to do this:

theres a form (with a drop down menu) with these options: A, B, C, and D

if A or B is selected, the form ends with the "continue" button

if C or D is selected, javascript kicks in and automatially makes an extra part of the form appear (which doesn happen with A or B) and the user needs to fill in another 2 mandatory text fields before they can click "continue" and have the form end

yeh, if they select C/D, the extra part appear, if they make the wrong selection and before they click "contineu", they select A/B, the extra part dissapears