Let's first assess the stimulus carefully (as always). This stimulus presents an argument, the conclusion of which is "pesticides are definitely contaminating the river." What are the premises given in support of this conclusion? (1) A local chemical plant produces pesticides that *can* cause sterility in small mammals such as otters, and (2) Shortly after this plant began operating, the incidence of sterility among the otters that swim in a nearby river increased dramatically.
This is plausible, but it's not airtight. Just because one cause for an effect exists does not definitively establish that this cause in fact led to this effect in this case. There may be other causes for the effect!
Answer choice (B) mimics this flaw. It is true that a diet low in calcium can cause a drop in egg production in poultry. But this does not necessarily mean that the food these chickens found and ate was necessarily undeniably low in calcium.
The flaw in answer choice (D) is an incorrect reversal. You are told that one necessary feature of apes is opposable thumbs (A ==> OT). You are then told that a fossil has been found with opposable thumbs, and that this fossil must therefore be an ape's. This is a different kind of logical flaw than assuming that one cause is the *only* cause.
Hope this helps! Please let us know if you have any additional questions.
