Question: In a 3GP file, how are enabled tracks handled by default, and what additional grouping allows for selective playback of alternative tracks?
Option 1: Enabled tracks are played sequentially by default; an 'alternate group' enables simultaneous playback of selected tracks.
Option 2: Enabled tracks are streamed simultaneously by default; no additional grouping is defined for handling alternative tracks.
Option 3: Enabled tracks are muted by default; a 'switch group' must be defined to enable playback of any track.
Option 4: Enabled tracks are streamed simultaneously by default; an additional 'alternate group' (with possible nested 'switch groups') allows for grouping alternative tracks for switching purposes.
Explanation: This correct answer reflects the nuanced understanding of how 3GP files handle multiple enabled tracks. By default, all enabled tracks are played simultaneously. However, to manage tracks that serve as alternatives to each other (e.g., different languages or video qualities), the ISO base media file format allows these to be grouped into an 'alternate group'. Furthermore, within this grouping, a 'switch group' can be defined to facilitate seamless switching between these alternative tracks during playback, enhancing user experience and flexibility.