Which of the following statements is NOT TRUE, according to the passage

A．Slaves in Greece in the sixth century B. C. were not politically significant.
B．The "burghers" of the later Middle Ages constituted an entirely new social class.
C．To some extent, social mobility can be attributed to the growth of trade.
D．The new class made up of traders and executive officials of the later Middle Ages made the development of a money economy possible.