The disk falls straight down into the glass when the index card is quickly pulled away due to inertia.
Inertia is the tendency of an object to resist changes in its state of motion. When the index card is pulled away quickly, the disk has inertia that keeps it in its original state of rest. Since the card is moving horizontally, the disk remains in place momentarily until the vertical force of gravity causes it to fall straight down into the glass.
So the correct response is: Inertia.
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