Answer:
The answer would be Point of Reference.
In order to determine, whether an object has moved or not, a point of reference is required. The reference point is required to the measure the movement of the object, without a reference point the distance or displacement cannot be obtained. Hence, a reference point is very required to determine, whether the object has moved or not.
(Hope this helps) Sky
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A. density
<span>Reaction rate increases with concentration.</span>
I think its the mirror because condenser helps in allowing the amount of light to pass.