S1: To most people, the term technology conjures up images of mills or machines. P: The classic symbol of technology is still the assembly line created by Henry Ford half a century ago. Q: The invention of the horse collar in the Middle Ages led to changes in agricultural methods and was as such a technological advance. R: Moreover, technology includes techniques, as well as the machines that may or may not be necessary to apply them. S: This symbol, however, has always been inadequate, for technology has always been more than factories and machines. S6: It includes ways to make chemical reactions occur, ways to breed fish, plant forests or teach history. Find the correct sequence of P, Q, R and S.