Two events occur simultaneously on the x axis of reference frame S, one at x = −a and the other at x = +a. According to an observer moving in the positive x direction:

A. the event at x = +a occurs first
B. the event at x = −a occurs first
C. either event might occur first, depending on the value of a and the observer’s speed
D. the events are simultaneous

The correct answer is A.
Why is it not B?

To determine which event occurs first, we need to consider the perspective of the observer moving in the positive x direction.

In this scenario, the observer is moving towards the positive x direction, which means they are approaching the event at x = +a and moving away from the event at x = -a.

Since the observer is moving towards the event at x = +a, they will reach and pass that event before reaching the event at x = -a. Therefore, from the perspective of the observer, the event at x = +a occurs first.

On the other hand, from the perspective of an observer moving in the negative x direction, they would see the event at x = -a occurring first. The frame of reference and the direction of motion affect the order in which the events are observed.

Hence, the correct answer is A because it is stated that the observer is moving in the positive x direction.