What is the defining feature of a system?   a. properties that don’t change  b. collision   c. interaction   d. gravitational force

The correct option is C. 
A system is defined as a collection of components or elements which are dependent on one another and are working together to form a complete unit. The defining feature of a system is interaction; this is because the components of the system are interacting together in order to perform their expected functions.

For me a person with a phd and who knows what there talking about.i know the chances of a radiation leak is low but if it does the area within a 96.3 mile radius would be unusable because of contamination