The answer would be 20 because you have 5 blocks that are cut into 1/4. It doesn't say anything about any of the pieces disappearing or being used yet so you multiply 4*5 and then you get the answer.
Selection B is appropriate.
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The signs of the coefficients tell you the binomial factors must have both signs positive (selections B and C only). Selection B produces an x-term that is 15x+15x=30x (as we want); selection C produces an x-term that is 45x+5x=50x (not what we want).