先是AA

11. The following appeared in the editorial section of a local newspaper.

“In the first four years that Montoya has served as mayor of the city of San Perdito, the population has decreased and the unemployment rate has increased. Two businesses have closed for each new business that has opened. Under Varro, who served as mayor for four years before Montoya, the unemployment rate decreased and the population increased. Clearly, the residents of San Perdito would be best served if they voted Montoya out of office and reelected Varro.”

地方性报纸的编者按：
在Montoya当San Perdito市长的头4年里，人口下降而失业率上升。每当一个新企业开业，就有两个旧的关门。而在Montoya之前Uarro当市长的4年里，失业率下降人口增长。很明显，San Perdito的居民重新选举Varro而非Montoya会得到更好的服务。

有一道很简单的题，买了六本不同的书，出差时带四本，有多少种带法？