What is the largest city in North America?  What is the smallest city in North America?  What is the distance from California to London?  What is the width of the Earth?  What is the distance from the Earth to Mars?  What was the last planet we have discovered?  How far away is the last planet we discovered?
> What is the largest city in North America?

The largest city (by population) in North America is Mexico City with more than 9 million people.


> What is the smallest city in North America?

It's hard to answer that question definitively. It depends on what definition one uses for city. There are some towns with only a single resident and even ghost towns, cities, villages without anyone living there.


> What is the distance from California to London?

The linear distance between California and London (including Earths curvature) is 8 560 km (5 320 mi).


> What is the width of the Earth?

Keep in mind that the Earth is not a perfect sphere. It's diameter is roughly 12 750 km (7 920 mi)


> What is the distance from the Earth to Mars?

The distance between Earth and Mars depends on where those planets are currently are in their orbit around the Sun. At it's closest the distance would be about 55 million km (34 million mi) and at it's farthest 400 million km (250 million mi). On average the distance is about 250 million km (140 million mi).


> What was the last planet we have discovered?

The last planet that was discovered in our solar system was Neptune in September 1846. Humans also have discovered planets in other solar systems (so called exoplanets) more recently.


> How far away is the last planet we discovered?

Neptune is roughly 4.5 billion km (2.8 billion mi) away. Like with all the other planets the current exact distance oscillates depending on various factors.