A、The new technology used to build roads. B、The ability to transport goods across the world. C、The fast growing trade in grain a

Good afternoon, everybody, today I am going to tell you some information about agricultural trade in the United States in the past.(16)About 200 years ago, the United States economy was growing quickly, mainly because a booming trade in grain and cotton. This trade went on near areas that can be sold at the coast, or near rivers that can run boats. It took place there because it was so expensive to transport goods over the roads that existed. They were muddy, narrow, and hard to travel on. At that time, don’t forget there was only one continuous road that existed in the US, and it was made up of north-to-south local country roads, linked together to get one long road. Within a short time, the first east-to-west roads were built. They were some like the present highways.(17)Private companies built these roads, and collected fees from all vehicles traveled on them. These companies know how to make their own luck and that means being in the right place at the right time with the right tools. Eventually, a network of roadways connected some major cities and towns. But even these roads were still very slow, traveling on them was too costly for farmers. They would spend much money to move their crops on these roads that they cannot afford it.(18)So, we see that even with some major improvement in roadways, farmers still had to rely on rivers to move their crops to markets.
16. What caused the growth of the US economy about 200 years ago?
17. What did private companies do after they built new roads?
18. What’s the passage mainly about?