Credit card, it's money they don't currently have but will be able to pay for in the future
The invention of the wheel by the Sumerians revolutionized the transportation. Wagons could be used to carry heavy loads. The Tigris and Euphrates rivers made transport of goods easy and economical. Riverboats were used to transport goods for trade. Strong currents moved the boats downstream, but because of the current they could travel in one direction only. The boats had to be dismantled after the trip downstream. The Mesopotamians used three types of boats: wooden boats with a triangular sail, the turnip or Guffa boat which was shaped like a tub, made of reeds and covered with skin, and the kalakku which was a raft of timbers supported by inflated animal skins.
Sometimes they figure out how to build better shelters or learn better farming and hunting techniques for the environment they live in
I would say d. division of the empire into 2 halves.
I believe that although the constitution has not changed much since it was written, but I do believe that however, the interpretation of the constitution has changed over time. I believe that the Founding Fathers would be a little shocked if they were to travel through time and see how the US is right now.
