Value List is allowing selection of more than one item in the list
I have a number of value lists on my form, and it appears that the user can select multiple entries from the list. (In windows holding down the control key and selecting them from the drop down list.
These are remembered, and returned . See attached screen shot. Items Aerial Operations with rope and harness, ground operations using Hiab and Stump treatment, are three items from the value list.
I only want the user to be able to select one item from the value list. How do I restrict the user to only being able to select one item from the value list, can see no option when defining this control.