I think you forgot to add the options along with the question. I am answering the question based on my research and knowledge. "Westward expansion" is the one reason that <span>led to the most sectional strife over slavery in the first half of the 19th century. I hope that the answer has actually helped you.</span>
it would be the development of Vaccine
For example, Before vaccine was invented, many people in Europe died from small pox. Approximately, small pox alone took around 400,000 of European people each year. (to understand how bad it is, it's important to know that their total population at that time was so small compared to the current population)
The development of vaccine prevented European people during industrial revolution from infected from many type of illness that could easily harm them due to lack of knowledge to treat them.
It forced him to go through the motions of making treaties with the tribes.
Investment practices that most destabilized the stock market. Stock speculation and buying on margin made the stock market very unstable. Investors made risky investments with very little money to back them up, and the stock market became overvalued.
I think it is A
Hope this help you!!!!!???? I think it is D