A. Because you don't have any Apple Loops in those categories (or no loops in those categories that fit the key and/or time signature of your project). If there's nothing to show you, GarageBand disables the button.
GarageBand from Apple Computer uses audio loops and MIDI loops in Apple Loops format. It can work with Audio Units plug-ins. Apple Macintosh Compatible FireWire and USB interfaces can be used to connect Audio and MIDI instruments to your Mac, and be recorded into GarageBand.
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