During the opening of the film Casablanca, we hear an airplane approaching and see people looking up at the sky; in the next shot, we see the airplane flying overhead. What continuity editing technique is primarily at work here?

Eyeline match.

Explanation:

Continuity Editing technique is demonstrated as the film or video editing method that is employed to integrate the shots to offer coherence and smoothness in the video or film.

Eyeline match is the type of continuity editing technique that aims to cut the two shots to create an illusion of the character who is watching on a screen in the first shot which is followed by a cut to display the thing or object he/she is looking/viewing at in the second shot. This

In the given example, the 'Eyeline match technique' is employed as the two shots are combined together where the 'people looking up at the sky' is displayed in the first shot and 'the flying airplane' in the second to show what they are viewing in the sky which helps offer connectivity among the two shots.