Firstly, let's figure out the side length of the smaller cubes. Since they have a volume of 125, and the formula for volume of a cube is V = s^3 (where s is the length of the sides) we know that the sides of the small cubes are 5. (This is because we recognize 125 as 5 cubed.)
Now, since the large cube has 4 small cubes to a side, we know the sides of the large cube are 20. Reapplying the volume formula, the volume of the large cube should be 20 cubed. The easiest way to do this arithmetic is to think of the 20 as 2x10 and cube them separately. Since 2 cubed is 8, and 10 cubed is 1000, we can multiply the two to get 20 cubed, getting us 8000. So the answer is D.
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