(单选题)

According to Para l, which of the following is true of space settlements?

A.They could prove to be a last resort for gaining life-sustaining supplies.

B.Their danger cannot be justified due to the risk to human life and high cost.

C.Having settlers staying on Mars is more appealing because of its potential as a colony.

D.The moon would provide an excellent training ground for future expeditions to Mars.

参考答案：D