Many people in the world live in big cities, which are often dirty and difficult places to live. So, some cities will get bigger. They will also get higher, and lower, because people will begin to live under the ground as well as in tall buildings. Scientists also say that men can begin to live in cities under the sea, when there is not enough space on the land. Of course, these new cities will be very expensive, and difficult to build, but they are not impossible. Already, some countries are building places to live under the ground so their people can go there if there is a new war. There are underground cities in Switzerland and China, built by the government to help some of the people live during a great war. But will people change if they live under the ground for a long time?For example some fish go blind if they live in the dark sea for a long time. People will not go blind, because there will be light underground, but they may change in some way. Scientists say that people who live in cities today are losing their senses of smell, touch and taste. They can't smell the dirty air, they can't taste the chemicals in their food. These senses are not as strong as before, when people lived in the country and grew their own food. The city has changed that. What will the underground city change in everyone? Some countries try to build some places under the ground so that people will be able to live there   _  .
when summer comes when a war breaks out because they think it'll be safer for them when they're fed up with the life on the land
Question: Many people in the world live in big cities, which are often dirty and difficult places to live. So, some cities will get bigger. They will also get higher, and lower, because people will begin to live under the ground as well as in tall buildings. Scientists also say that men can begin to live in cities under the sea, when there is not enough space on the land. Of course, these new cities will be very expensive, and difficult to build, but they are not impossible. Already, some countries are building places to live under the ground so their people can go there if there is a new war. There are underground cities in Switzerland and China, built by the government to help some of the people live during a great war. But will people change if they live under the ground for a long time?For example some fish go blind if they live in the dark sea for a long time. People will not go blind, because there will be light underground, but they may change in some way. Scientists say that people who live in cities today are losing their senses of smell, touch and taste. They can't smell the dirty air, they can't taste the chemicals in their food. These senses are not as strong as before, when people lived in the country and grew their own food. The city has changed that. What will the underground city change in everyone? Some countries try to build some places under the ground so that people will be able to live there   _  .
A. when summer comes
B. when a war breaks out
C. because they think it'll be safer for them
D. when they're fed up with the life on the land
Answer: