两道princeton review模考上的语法

1. innovations in production techonology and decreases in the cost of equipement have made recycling paper into new paper products much more cost-efficient over the last twenty years. Despite these advances, though, the "point of price viability"(the price that new paper made from trees must reach to make recycled paper comparable in price) is unchanged at $2.12 per ream of paper

which of the following ,if true, most explains why the incresed cost-effciency of recycled paper has not lowered the point of price viability?

B.the decreases in the cost of recycling equipment yhave occurred despite increases in the cost of raw materials required to manufacture such equipment

C.innovations in production techonology have made it much more cost-effcient to produce new paper from trees

我选的是B，答案是C

谁能把这个题目的逻辑链说一下吗

2. a recent stucy of college sutdnets shows that, contrary to predicted results, special nutritional planning does not positively affect students'grades. sixty students, half of whom were given a nutritionally balanced diet, had grades no higher than did those students who were not placed on the diet plan

which of the floowing , if ture, is most useful in determining the accuracy of the study decribed above?

B.honors students, after altering their diets, maintained that they did not change their study habits

D. high school students who previously had low grades found that after they altered their diets, their grades improved dramatically

答案是D，我选的是B，我觉得B可以排除他因啊，就是说study habits没有影响到结果

拜托大家了，我下周就考试了，谢谢了哦~~