单选题 During the Middle Ages, productivity of labor ______.

A、 was a primary concern of society
B、 was hampered by class relationships
C、 began to improve over levels reached by the Greeks
D、 was in a period of technical progress

【正确答案】 B

【答案解析】可在第四句话“The social system of fixed... the Middle Ages (and in some areas much longer) itself hampered improvement.”找到。A社会焦点；C超过了希腊研究高度；D技术进步时期；均不符文意。