Answer: Choice B)

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Step-by-step explanation:
The domain is the set of allowed x inputs of a function.
How far to the left can we go on this red line? That would be x = 7.
How far to the right? That would be x = 12.
We can have x equal anything between 7 and 12, including both endpoints. We include the endpoints because they are filled in circles (as opposed to open holes).
So the domain is
. If we replace x with r, then we update that inequality into
. This replacement happens because r is along the horizontal x axis.