Which of the following statements is true?

A. Under conventional costing, product cost distortions will be significant if overhead costs are a large proportion of product cost.

B. Activity-based costing will significantly improve the product cost accuracy where overhead costs are a small proportion of total costs.

C. Activity-based costing will not significantly improve product cost accuracy where there is significant product diversity.

D. Upstream and downstream costs are often an insignificant part of a product's costs.