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5.     Animals with a certain behavioral disorder have unusually high level of aluminum in their brain tissue. Since a silicon-based compound binds to aluminum and prevents it from affecting the brain tissue. Animals can be cured of the disorder by being treated with the compound.

The argument is based on which one of the following assumptions?

(A) Animals with the disorder have unusually high but invariable levels of aluminum in their brain tissue.

(B) Aluminum is the cause of the disorder rather than merely an effect of it.

(C) Introducing the compound into the brain tissue has no side effects.

(D) The amount of the compound needed to neutralize the aluminum in an animal’s brain tissue varies depending upon the species.（B）

(E) Aluminum is never present in normal brain tissue

为什么不选A?

C 项 的merely 是说铝是原因,不是结果:high level  aluminum--à disorder ? 充分条件

还是:铝既是原因又是结果: high level  aluminum<--à disorder 充分必要条件

i cannot answer your question in chinese.

1. there are no sufficent and necessary conditions in the stimulus because of no indicator, such as if, only

2. the answer B rule out the possibility of reverse cause and effect