Final answer:
Possible factors for the decline in average life expectancy in the US include lifestyle factors, healthcare issues, and the COVID-19 pandemic.
Step-by-step explanation:
There could be multiple underlying reasons for the decline in average life expectancy in the US for three consecutive years (2015-2016, 2016-2017, 2017-2018). One hypothesis to explain this trend is the impact of lifestyle factors such as rising poverty, unhealthy habits, and associated diseases. Another hypothesis is the influence of healthcare issues, including access to quality healthcare and healthcare expenditure. Additionally, the unprecedented COVID-19 pandemic could have played a significant role in this decline, as it has caused a substantial number of deaths worldwide.