【单选题】

{{B}}{{I}} Question 21 and 22 are based on the following news.At the end of the news item, you will be given 10 seconds to answer the question. Now listen to the news.{{/I}}{{/B}} The cause of the riot was ________.

A、a dispute over working hours
B．a dispute over wages
C．a dispute over working conditions
D．a dispute over safety measures