nakib77 wrote:
The milk of many mammals contains cannabinoids, substances that are known to stimulate certain receptors in the brain. To investigate the function of cannabinoids, researchers injected newborn mice with a chemical that is known to block cannabinoids from reaching their receptors in the brain. The injected mice showed far less interest in feeding than normal newborn mice do. Therefore, cannabinoids probably function to stimulate the appetite.
Which of the following is an assumption on which the argument depends?
A. Newborn mice do not normally ingest any substance other than their mothers' milk.
B. Cannabinoids are the only substances in mammals' milk that stimulate the appetite.
C. The mothers of newborn mice do not normally make any effort to encourage their babies to feed.
D. The milk of mammals would be less nutritious if it did not contain cannabinoids.
E. The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.
To know the function of Cs, researchers injected newborn mice with a chemical that is known to block Cs from reaching their receptors in the brain.
The injected mice showed far less interest in feeding than normal newborn mice do.
Therefore, Cs probably function to stimulate the appetite.
Researchers injected a chemical that blocks Cs. The injected mice stopped feeding. So we are concluding that Cs stimulate feeding.
But what if the injected chemical is inhibiting feeding? Then our conclusion cannot hold. So we are assuming that the injected chemical does not inhibit feeding.
A. Newborn mice do not normally ingest any substance other than their mothers' milk.
Irrelevant. The argument is all about feeding. Injected mice showed far less interest in feeding than normal newborn mice do.
B. Cannabinoids are the only substances in mammals' milk that stimulate the appetite.
No we are not assuming that Cs are the only substances that stimulate appetite. Perhaps there are others too. We have seen a REDUCTION in appetite. Blocking Cs using a chemical is causing reduced appetite. So we are concluding that Cs do play some role in stimulating appetite.
C. The mothers of newborn mice do not normally make any effort to encourage their babies to feed.
Irrelevant. The point is about the interest of newborns in feeding.
D. The milk of mammals would be less nutritious if it did not contain cannabinoids.
Out of scope. We are not assuming anything about nutrition.
E. The chemical that blocks cannabinoids from stimulating their brain receptors does not independently inhibit the appetite.
Correct. As discussed before, we are assuming that the chemical we are injecting is itself not causing the reduction in appetite. The Cs may have no role in appetite if the chemical is the one causing reduction.
Answer (E)