Growing the same crop over and over soil.

the great wall of china worked as a defense for the feudal states of the zhou dynasty to the north. during the warring states period more walls were put up to defend the borders of the different kingdoms. it was semi successful because despite the wall, the mongols eventually conquered china also the great wall provided protection to the economic development and cultural progress, safeguarded the trading routes such as the silk road, and secured transmission of information and transportation. @jayxboof

yellow fever and other tropical diseases

the answer to your question is a. the salt plains

in 1811, he led an expedition, known as the george c. sibley expedition, to improve relations with the pawnee and kansa tribes, and also to locate the rumored jefferson's salt mountain

from thales, who is often considered the first western philosopher, to the stoics and skeptics, ancient greek philosophy opened the doors to a particular way of thinking that provided the roots for the western intellectual tradition. here, there is often an explicit preference for the life of reason and rational thought. we find proto-scientific explanations of the natural world in the milesian thinkers, and we hear democritus posit atoms—indivisible and invisible units—as the basic stuff of all matter. with socrates comes a sustained inquiry into ethical matters—an orientation towards human living and the best life for human beings. with plato comes one of the most creative and flexible ways of doing philosophy, which some have since attempted to imitate by writing philosophical dialogues covering topics still of interest today in ethics, political thought, metaphysics, and epistemology. plato’s student, aristotle, was one of the most prolific of ancient authors. he wrote treatises on each of these topics, as well as on the investigation of the natural world, including the composition of animals. the hellenists—epicurus, the cynics, the stoics, and the skeptics—developed schools or movements devoted to distinct philosophical lifestyles, each with reason at its foundation.

with this preference for reason came a critique of traditional ways of living, believing, and thinking, which sometimes caused political trouble for the philosophers themselves. xenophanes directly challenged the traditional anthropomorphic depiction of the gods, and socrates was put to death for allegedly inventing new gods and not believing in the gods mandated by the city of athens. after the fall of alexander the great, and because of aristotle’s ties with alexander and his court, aristotle escaped the same fate as socrates by fleeing athens. epicurus, like xenophanes, claimed that the mass of people is impious, since the people conceive of the gods as little more than superhumans, even though human characteristics cannot appropriately be ascribed to the gods. in short, not only did ancient greek philosophy pave the way for the western intellectual tradition, including modern science, but it also shook cultural foundations in its own time.