Answer:
a) DeGrasse Tyson's metaphor in paragraphs 8-11 is that of a river. He compares the progress of scientific knowledge to the meandering of a river, which slowly but surely finds its way to its destination.
b) The use of this metaphor serves to emphasize the inevitability of scientific progress. By likening scientific progress to the inevitable flow of a river, DeGrasse Tyson emphasizes that no matter how much resistance society puts up, science will eventually find its way to its destination. This metaphor also helps to illustrate the idea that science is something that slowly builds on itself, with each discovery leading to the next. Thus, the use of this metaphor serves to strengthen DeGrasse Tyson's argument that scientific progress is both inevitable and beneficial.