装配线生产的优势，首先是速度快，其次是成本低。 A: The disadvantage of an assembly line are firsty speed and secondly the cost of the product. B: The advantage of an assembly line are firstly speed and secondly the cost of the product. C: The advantage of an assembly line are speed and the cost of the product. D: The advantage of an assembly line are either speed or the cost of the product.