Mônica Lopes-Ferreira and others at Brazil’s Butantan Institute are studying the freshwater stingray species Potamotrygon
rex to determine whether biological characteristics such as the rays’ age and sex have ______ effect on the toxicity of their
venom—that is, to see if differences in these traits are associated with considerable variations in venom potency.
a disconcerting
an acceptable
an imperceptible
a substantial
Choice D is the best answer because it most logically completes the text’s discussion of the research that Lopes-Ferreira
and her colleagues are conducting on the stingray species Potamotrygon rex. As used in this context, “a substantial” effect
means an effect that is sizable or noteworthy. The text indicates that the researchers are seeking to determine whether there
are “considerable variations” in the potency of stingray venom that are associated with variation in the stingrays’ age and
sex. This context suggests that the researchers want to ﬁnd out whether stingray age and sex have a substantial effect on
venom toxicity.
Choice A is incorrect because there’s nothing in the text that suggests that the researchers have been studying whether the
stingrays’ age and sex have “a disconcerting,” or an unsettling and disturbing, effect on the stingrays’ venom. The text
indicates that the researchers wish to determine if stingray age and sex cause large variations in the toxicity of stingray
venom, not if the effect of age and sex is disconcerting. Choice B is incorrect because the text indicates that researchers
want to ﬁnd out whether differences in stingray age and sex produce differences in stingray venom, not that the researchers
want to ﬁnd out whether age and sex have “an acceptable,” or a satisfactory, effect on venom. The text makes no mention of
what would make an effect on venom toxicity acceptable and gives no indication that the researchers are interested in that
question. Choice C is incorrect because it wouldn’t make sense in context for the researchers to be looking for “an
imperceptible,” or an unnoticeable, effect of age and sex on stingray venom. The text says that the researchers are trying to
determine if there are “considerable variations” in venom toxicity linked to age and sex, not that the researchers are trying to
ﬁnd effects that they can’t perceive.