I think the answer is d not a 100 % sure
Believe it's... people working in midday.
Japan is a small isle chain with millions of people due to the fact it is mainly arable (It allows for agriculture), but the fact is the increase technology brings up the population of Japan. Japan is also still influx of population by World war 2, as they had MILLIONS upon MILLIONS die on different isles, lands, etc. They also had a major emigration after WWII.