A batch of new products are not up to standard. After inspection, the reason is not due to the poor quality of raw materials or the design error of the products. From this we conclude that the sub-standard products are due to the low skill level of the workers.
The statement "The product does not meet the requirements, either because of the poor quality of raw materials, or because of product design errors, or because of the low level of skill of the workers" best supports the conclusion.