Mary, a veterinary student, has been assigned an experiment in mammali

I think this question has already been discussed in some previous posts, please search before posting to avoid duplication.

In case you feel not like to do that, here is my reasoning.

(A) All other things being equal, gratuitously causing any animal to suffer pain is unjustified.
The word "gratuiously" contrasts with the stimulus, which clearly states that this experiment is done for mammalian physiology

(D) Practicing veterinarians have a professional obligation to strive to prevent the unnecessary death of an animal except in cases of severely ill or injured animals whose prospects for recovery are dim.
Mary is a vet student, not practicing.

(E) No one is ever justified in acting with the sole intention of causing the death of a living thing, be it animal or human.
The sole intention of this experiment is not to kill a living thing, but for a study in mammalian physiology.

I leave (B) and (C) to compare, because their meanings are quite similar. Look at the main difference here:
(B) Taking the life of an animal is not justifiable unless doing so would immediately assist in saving several animal lives or in protecting the health of a person.
We know that the experiment cannot immediately support animal lives, but cause them to death.

(C) The only sufficient justification for experimenting on animals is that future animal suffering is thereby prevented.
We cannot tell whether the experiment will help prevent animal suffering in the future, because it does not mentioned in the stimulus. It may/may not.

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