单选题 Firms adopting "profit-related-pay"(PRP) contracts pay wages at levels that vary with the firm's profits. In the metalworking industry last year, firms with PRP contracts in place showed productivity per worker on average 13 percent higher than that of their competitors who used more traditional contracts.
If, on the basis of the evidence above, it is argued that PRP contracts increase worker productivity, which of the following, if tree, would most seriously weaken that argument?

A、 Results similar to those cited for the metalworking industry have been found in other industries where PRP contracts are used.
B、 Under PRP contracts costs other than labor costs, such as plant, machinery, and energy, make up an increased proportion of the total cost of each unit of output.
C、 Because introducing PRP contracts greatly changes individual workers' relationships to the firm, negotiating the introduction of PRP contracts is complex and time consuming.
D、 Many firms in the metalworking industry have modernized production equipment in the last five years, and most of these introduced PRP contracts at the same time.
E、 (E) In firms in the metalworking industry where PRP contracts are in place, the average take-home pay is 15 percent higher than it is in those firms where workers have more traditional contracts.

【正确答案】 D

【答案解析】[解析] 本题应把问题目的与段落结合起来进行思雏，且论点包含于问题目的中。论点是：PRP提高了工人的劳动生产率；论据是“金属行业采用PRP比其他行业工人生产率高13%。当由数据得出一个对数据的解释时，反对一般用“有其他原因”的思路。(D)指出金属工业行业生产设备现代化了，其效率提高完全有可能是由于设备现代化的原因，从而削弱了上面的论点，所以(D)正确；(E)易误选，但(E)只表明税后支付高，至于与生产率高有什么关系不知道，换句话说，它并没有说明究竟是高工资导致了高生产率，还是高生产率导致了高工资，因此(E)是无关答案。
注：take-home pay税后工资；实领工资