The correct answer is d. a and b.
Both statements in options a and b are valid reasons why global warming might lead to the spread of insect-borne diseases:
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a: Warmer temperatures, particularly milder winters, can result in higher insect survival rates, allowing more insects to survive and reproduce, which can lead to an increased presence of disease vectors.
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b: Warmer temperatures can expand the habitat range of certain insects (such as mosquitoes and ticks) into regions that were previously too cold for them to thrive, thereby introducing the associated diseases to new populations.
Option c is also true, as warmer temperatures indeed make bugs more active, but it's more about their behavior rather than their geographic spread. However, since both a and b are explicitly about the impact on the spread of diseases, d is the most comprehensive choice.