The next great land area that man hopes to control is the moon. In size it is nearly equal to the area of North and South America. However, it presents a hostile   environment. Temperatures range +120 to -150 degrees Centigrade. There is no air, and no water. Today there is considerable scientific speculation   about living on the moon. When man will begin life on the moon surface is still not determined. But experts believe that settlement will take place in three steps. First, there will be increasing periods of exploration with temporary shelters. These periods will be followed by longer stays with housing under the surface of the moon and daily necessities brought by the settlers themselves from the earth. Finally, settlements that are self-supporting will be founded. The main job of the early settlers will be to stay alive. They will have to plant crops under huge domes   to produce food and oxygen and find water sources. After this is done, the settlers will have time to explore the possibilities of commercial   development and to make discoveries important to science. The characteristics of the moon that make it bad for human staying alive may make it ideal for certain kinds of manufacturing . Operations requiring a vacuum   or extreme cold are examples. Therefore, industrial diamonds might be produced on the moon. To stay alive on the moon, first of all the early settlers must be able to    _   .
get enough food, oxygen and water develop commerce make discoveries important explore the possibilities of industrial development
Question: The next great land area that man hopes to control is the moon. In size it is nearly equal to the area of North and South America. However, it presents a hostile   environment. Temperatures range +120 to -150 degrees Centigrade. There is no air, and no water. Today there is considerable scientific speculation   about living on the moon. When man will begin life on the moon surface is still not determined. But experts believe that settlement will take place in three steps. First, there will be increasing periods of exploration with temporary shelters. These periods will be followed by longer stays with housing under the surface of the moon and daily necessities brought by the settlers themselves from the earth. Finally, settlements that are self-supporting will be founded. The main job of the early settlers will be to stay alive. They will have to plant crops under huge domes   to produce food and oxygen and find water sources. After this is done, the settlers will have time to explore the possibilities of commercial   development and to make discoveries important to science. The characteristics of the moon that make it bad for human staying alive may make it ideal for certain kinds of manufacturing . Operations requiring a vacuum   or extreme cold are examples. Therefore, industrial diamonds might be produced on the moon. To stay alive on the moon, first of all the early settlers must be able to    _   .
A. get enough food, oxygen and water
B. develop commerce
C. make discoveries important
D. explore the possibilities of industrial development
Answer: