Answer:
3. D. will be
4. A. would have had
Step-by-step explanation:
*3. If the level of the sea rises, villages by the sea will be flooded.
This is because the rising sea level will cause the water to inundate the land that is currently below sea level. This will mean that the villages by the sea will be underwater, and they will no longer exist.
*4. If there were no criminals, the police would have had nothing to do.
This is because the police are responsible for investigating and apprehending criminals. If there were no criminals, then the police would not have any work to do.
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The answer for 3 could also be C. will not be, but this would be a less likely outcome. If the sea level rises slowly enough, then the villages by the sea may be able to adapt to the rising water by building seawalls or elevating their homes. However, if the sea level rises rapidly, then the villages by the sea will likely be flooded and destroyed.
The answer for 4 could also be B. will have had not been, but this would be a less accurate way of expressing the idea. The police would not have had not been doing anything, they would have simply been doing nothing. The answer would have had is more accurate because it expresses the idea that the police would have been available to do work, but there would have been no work for them to do.