mymba99 wrote:
In polluted environments, dolphins gradually accumulated toxins in their body fat, and the larger the dolphin the more accumulated toxin it can tolerate. Nearly 80 percent of the toxins a female dolphin has accumulated pass into the fat-rich milk her nursing calf ingests. Therefore, the unusually high mortality rate among dolphin calves in the industrially contaminated waters along Florida’s Gulf Coast is probably the result of their being poisoned by their mother’s milk.
Which of the following, if true, most strengthens the argument?
(A) The survival rate of firstborn dolphin calves in the area along Florida’s Gulf Coast is highest for those whose mothers were killed before they were weaned.
(B) The rate at which adult dolphins living in the waters along Florida’s Gulf Coast accumulate toxins is no higher than that of adult dolphins in comparably polluted waters elsewhere.
(C) Among dolphin calves born in the area along Florida’s Gulf Coast, the mortality rate is highest among those with living siblings.
(D) As dolphins age, they accumulate toxins from the environment more slowly than when they were young.
(E) Dolphins, like other marine mammals, have a higher proportion of body fat than do most land mammals.
GMATPrep Code : VCR001296
Dolphins accumulate toxins in their body fat, and the larger the dolphin the more accumulated toxin it can tolerate. Nearly 80 percent of the toxins a female dolphin has accumulated pass into the fat-rich milk her nursing calf ingests.
There is unusually high mortality rate among dolphin calves in the industrially contaminated waters along Florida’s Gulf Coast.
Conclusion: It is probably the result of their being poisoned by their mother’s milk.
We need to strengthen that the reason for unusual mortality rate is mother's milk.
(A) The survival rate of firstborn dolphin calves in the area along Florida’s Gulf Coast is highest for those whose mothers were killed before they were weaned.
Correct. The infants survived better if they were fed on mother's milk for lesser time. Then it does seem more likely that they are being poisoned by mother's milk.
(B) The rate at which adult dolphins living in the waters along Florida’s Gulf Coast accumulate toxins is no higher than that of adult dolphins in comparably polluted waters elsewhere.
Irrelevant. Even if the rate of toxin accumulation is the same, we don't know what is the survival rate of calves in other polluted waters. Do they also see unusual mortality rate? We don't know. If they don't see unusual mortality rates then it seems that mother's milk is not responsible (that will weaken our conclusion). It they see unusual mortality rates too, then it doesn't mean that mother's milk is responsible. It could be the polluted waters that are directly making calves sick or some other such reason.
(C) Among dolphin calves born in the area along Florida’s Gulf Coast, the mortality rate is highest among those with living siblings.
We can't say what this means for our conclusion. If the calves have older siblings, they should get less toxins (since much of stored toxins could have already been passed on to the older sibling). Also, if a sibling is alive but this calf dies, mother's milk may not be the reason.
(D) As dolphins age, they accumulate toxins from the environment more slowly than when they were young.
Irrelevant. We are wondering why the calves' mortality has suddenly increased.
(E) Dolphins, like other marine mammals, have a higher proportion of body fat than do most land mammals.
No connect to mother's milk.
Answer (A)
jabhatta2An assumption is just a special kind of strengthener. All strengtheners (including assumptions) increase the probability of the conclusion but an assumption has the added characteristic of being necessary for the conclusion to hold.
Option (B) is not a strengthener so it can certainly not be an assumption.
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