Trade was a lot more difficult and complicated because every currency was worth something different.
Typically, the background of a study includes a review of the existing literature on the area of your research, leading up to your topic. Once you have discussed the contribution of other researchers in the field, you can identify gaps in understanding, that is, areas that have not been addressed in these studies.
Not sure if this is what you meant, if not, let me know
Answer:
D it causes river channels to change shape
