Improve userform using vba

Hi. I created a survey form using excel. But I will be very much pleased about it being fine-tuned for better administering of the survey and easier management of data results. I hope it will be modified according to the following conditions:

1. Multiple answers for each case is not allowed. That is, if the user chose ANSWER D for instance, then he/she will not be allowed to mark another answer altogether.
2. If ANSWER U is marked, user will be required to fill out columns U1 and U2.
3. User will not be able to close the excel form if not all cases have been completely answered. Likewise in the case where User chose ANSWER U, he/she will not be able to close it until columns for U1 and U2 have been filled out.
4. The form will be reproduced with completely different number of cases each copy (some can even reach up to more than 2,000), so I was hoping that the form will not be limited by the number of rows defined in the sample excel form.