Final answer:
There have been approximately 80 generations since the year 0 AD, calculated by dividing approximately 2020 years by the average length of a generation, which is about 25 years.
Step-by-step explanation:
The question asks to determine how many generations have passed since the year 0 AD given that a generation is about one-third of a lifetime. Since the current year is past 2020 AD, we can use an approximation to find the number of generations.
Let's consider the average human lifetime to be around 75 years. A generation would then be roughly 25 years (a third of a lifetime). Now, since it has been approximately 2020 years since the year 0 AD, we can divide 2020 by 25 to get the number of generations:
2020 years ÷ 25 years/generation ≈ 80.8 generations
Therefore, the closest answer would be (b) Approximately 80 generations.