Which of the following most logically completes the passage?

A certain tropical island received food donations in the form of powdered milk for distribution to its poorest residents, who were thought to be malnourished. Subsequently, the rate of liver cancers among those islanders increased sharply. The donated milk was probably to blame: recent laboratory research on rats has shown that rats briefly exposed to the substances aflatoxin tend to develop liver cancer when fed casein, a milk protein. This result is relevant because       .

1. in the tropics, peanuts, a staple of these island residents, support a mold growth that produces aflatoxin.

2. the liver is more sensitive to carcinogens, of which aflatoxin may be one, than most other bodily organs.

3. casein is not the only protein contained in milk.

4. powdered milk is the most appropriate form in which to send milk to a tropical destination.

5. the people who were given the donated milk had been screened for their ability to digest milk.

请问为什么是答案A？A中都没有涉及powdered milk

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