For this case, the switch is located at point B of the diagram.
Remember that point D is the universal symbol for resistance.
In A what you have is a source of power and in C what you have is a cable.
Therefore, the answer for this case is B.
Density can be any number, as long as it has the right units.
A unit of density has to be (a unit of mass) divided by (a unit of volume).
The most common one is gram/cm^3.
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