It was in the January category.
The asteroid belt is between Mars and Jupiter. Pluto is not in this location, nor is the Kuiper Belt.
This is moderately confusing to me, too. If you count the 12 days of Christmas (incidentally, probably why Alisha guessed Jan. 12) with Christmas as day #1 (which it is), the 12th day of Christmas is Jan. 5. But Epiphany is the next day, and I tend to think of Epiphany as the 12th day of Christmas. But I guess it's not.
I went with nonproliferation instead of disarmament, but it's too long. (I guess, technically, nonproliferation is about not allowing new countries to get nukes, and disarmament is about having countries with nukes to give them up. So I guess it would not have been accurate, anyway.) I'm not a huge fan of categories that require words to be of a certain length, because if I have a guess, I'm not going to take the time to count out the letters.
Nigeria has more than twice that number of people. It has about 190 million people. It is not just the most populous country in Africa (by a long shot), it is the 7th most populous country in the world. (China, India, US, Indonesia, Brazil, Pakistan, Nigeria, Bangladesh, Russia, Japan are the top 10.) Lagos, the former capital of Nigeria, is the 4th largest city (proper) in the world, 24th largest urban area in the world, and 12th largest metropolitan area in the world. (So it's a big city, and its world ranking depends on how you want to define city.)
Ethiopia, the second-most populous country in Africa, has about 105 million. (My guess of South Africa for #2 only has 54 million.)