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Answer (B) is correct . Lower-skilled workers tend to be less efficient, which would give rise to an unfavorable labor efficiency variance and possibly an unfavorable materials quantity variance.

Answer (A) is incorrect because The use of higher quality materials would explain a favorable material quantity variance but would tend to generate an unfavorable labor efficiency variance since higher quality materials require more careful handling and the workers would not be accustomed to working with those materials.
Answer (C) is incorrect because More efficient machinery would tend to generate an unfavorable fixed overhead volume variance since the purchase resulted in higher fixed costs.
Answer (D) is incorrect because A decrease in production supervision would lead to less efficiency among laborers since there would be no one watching over their work.