10. In the theory of relativity, simultaneity of events in different inertial frames is: A. Always observed C. relative to the observer B. Never observed D. Dependent on gravitational fields

The correct answer is C. relative to the observer.<br /><br />In the theory of relativity, simultaneity of events in different inertial frames is relative to the observer. This means that two events that are considered simultaneous in one inertial frame may not be considered simultaneous in another inertial frame. The theory of relativity states that the simultaneity of events is not an absolute concept, but rather it depends on the relative motion of the observer and the events. Therefore, the correct answer is C. relative to the observer.