Making a worksheet

Hello. I am trying to figure out how I can have a pull down menu type question where students can choose multiple correct answers? Can you please explain to me how to do this?

Ex:  Which three factors of a battery can be modified to maximize the voltage of the battery?  

        the combination of electrodes

        the intensity of the chemical reaction

        the surface area of the electrodes

        arrange the cells in series

        arrange the cells in parallel

I would like for students to be able to check three of the above factors. I have a picture of what I have done below but students can only check one correct answer. 

Thank you!