In this task, you're given a question, a context passage, and four options which are terms from the passage. After reading a passage, you will get a brief understanding of the terms. Your job is to determine by searching and reading further information of which term you can answer the question. Indicate your choice as 'a', 'b', 'c', or 'd'. If you think more than one option is plausible, choose the more probable option to help you answer the question.
Q: Question: What are earthwork mounds made of? Passage:Indigenous peoples inhabited Missouri for thousands of years before European exploration and settlement. Archaeological excavations along the rivers have shown continuous habitation for more than 7,000 years. Beginning before 1000 CE, there arose the complex Mississippian culture, whose people created regional political centers at present-day St. Louis and across the Mississippi River at Cahokia, near present-day Collinsville, Illinois. Their large cities included thousands of individual residences, but they are known for their surviving massive earthwork mounds, built for religious, political and social reasons, in platform, ridgetop and conical shapes. Cahokia was the center of a regional trading network that reached from the Great Lakes to the Gulf of Mexico. The civilization declined by 1400 CE, and most descendants left the area long before the arrival of Europeans. St. Louis was at one time known as Mound City by the European Americans, because of the numerous surviving prehistoric mounds, since lost to urban development. The Mississippian culture left mounds throughout the middle Mississippi and Ohio river valleys, extending into the southeast as well as the upper river.
Links: a. Archaeology b. Collinsville, Illinois c. Earthworks (archaeology) d. Collinsville, Illinois
A:
c