Final answer:
To select all slides, click on the first slide, press the Shift key, and then click on the last slide.
Step-by-step explanation:
To select all slides, you should proceed by clicking on the first slide in the Slides Pane, then press the Shift key and then click on the last slide. This will highlight all the slides in between. Using the Ctrl key with the first and last slide will only select those two slides.