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. - Irrelevant
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. - Irrevelant
D. The milk of mammals would be less nutritious if it did not contain cannabinoids. - Irrelevant
E. The chemical that blocks cannabinoids from stimulating their brain receptors does
not independently inhibit the appetite.

P: The injected mice showed far less interest in
feeding than normal newborn mice do.
C: Therefore, cannabinoids probably function to stimulate the appetite.

Now B and E seem equally likely here.
If I weaken E, i.e the chemical independently inhibits the appetite, then it puts the Conclusion in jeopardy because there are other substances too that can act like C.

If I weaken B, cannabinoids are not the only substances, there are other substances too that inhibit the appetite: then the argument stays because the word probably in mentioned in the Conclusion.

Please HELP!!
Also mention the approach you use for Assumption Q's.
Thanks

In general, on assumption questions, you should be able to insert the correct answer as a premise in the argument and have it strengthen the argument. Note also the language of the question: it says that the argument depends on it. It has to be necessary for the author to believe this in order to draw the conclusion s/he draws.

I'll only address choices B and E here, since it seems like you were fine with the others. B has an immediate red flag: the word "only." In general on CR and RC, extreme statements are wrong.

The author does not have to assume that cannabinoids are the ONLY substances that stimulate the appetite - just that they are a substance that stimulates the appetite.

By contrast, E must be true. The experiment introduces a new chemical into the brain. If we're going to conclude that the absense of the cannabinoids is what decreased the appetite, we MUST assume that this chemical we injected into their brains wasn't responsible for that decreased appetite itself.

Only E, if I insert it into the argument, is absolutely necessary. B takes it too far with the word "only."

Thanks.
I agree with the extreme wording.
In this case it was easier since we had the work ONLY.
Otherwise, it takes a monster attempt to break through.