I would say that sand in water is a suspension as you can see the particles with the naked eye or a microscope. Also, the solution will settle if left alone and the sand will sink to the bottom.
The independent variable is the different amount of repellent each person is given. This is because it does not depend on any variable. The dependent variable is what they are measuring, which would be the number of bites on each volunteer. I hope this helps! :)
