In areas with limited access to motorized transportation, farmers plant crops as close as possible to their homes to minimize the walking required to reach their fields and the work required to transport their harvest back to their homes. Farmers have almost no motorized transportation in certain parts of Africa where cassava is an important food crop. Although by no means the easiest to transport, cassava is invariably the crop planted farthest from these farmers' villages.
Which of the following, if true, provides the best explanation for the relative placement of the cassava crops?
2. 在文段的最后，会写：since/because ______.
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 substance aflatoxin tend to develop liver cancer when fed casein, a milk protein. This result is relevant because __________.