How to create a multi-item selection combobox in csharp

I am creating a students report card in windows forms,crystal reports and access database,

i have a form where someone can choose which exam to generate a report card for i.e Exam1,exam2,exam e.t.c using a drop down list and that works perfectly.Now i am trying to create a way of generating a report for all the exams where by the average marks obtained from each subject is calculated to get a final figure.My question is how do i come up with a multi-selection combobox so that all the selected exams` marks get printed on the report card.

Here is the report card template i have designed using crystal reports#