How to select measurement objects
First click the Select button 
.
To select a single object:
- Click on the object in the image.
To select more objects:
- Click on different objects while holding down the Ctrl key.
- or -
- Drag a rectangle over the different objects.
To unselect objects:
- Click on the objects while holding down the Ctrl key.
- or -
- Drag a rectangle over the objects while holding down the Ctrl key.