正确答案： C

解析：
录音提到“Unfortunately, in local jails, criminals committing all levels of offense are housed in the same facilities and are often separated to avoid violence ...”以及“The LA County jail has a history of prisoner-on-prisoner murder”，由此可知地方监狱监禁着的各种犯罪分子比联邦监狱或州立监狱监禁着的犯罪分子更危险。
【听力原文】
　　Now, as we discussed last class, our current system of prisons is a legacy of the history of the nature of penal reform and the attempts at social reform at all levels of society in the 1960s. Currently, we have three types of prisons: federal, state, and local. Federal prisons house the most dangerous criminals and those who have committed serious crimes against the government. State prisons have different levels, with maximum security housing violent offenders such as murders and rapists while minimum security prisons house large numbers of white collar and minor offenders.
　　Local jails, sometimes called county jails, house people awaiting trial and those serving very short sentences for minor infractions. Unfortunately, in local jails, criminals committing all levels of offense are housed in the same facilities and are often separated to avoid violence ... This isn’t always successful. An example would be the Los Angeles County jail, which is notorious for being divided along racial lines. White, black, Hispanic, and Asian prisoners are segregated, not by official policy, but by a jailhouse policy established by the prisoners themselves. Each group has a “shot caller”, a de facto leader whose job is to represent his group in any negotiations with the others in order to keep the peace or, at times, go to war. The LA County jail has a history of prisoner-on-prisoner murder, most of which is related to the rival street gangs of the city. While crime in the nation has gone down, our prisons still have one of the largest inmate populations in the world. Between 1990 and 2000, the number of prisoners has almost doubled from 1.2 million to just over two million. It may not surprise you that most of the nation’s prisoners are males. Females account for only six percent of all prisoners. In addition, the prisoners are not representative of America’s social groups. Blacks and Hispanics comprise two thirds of the prisoners yet are less than one third of the nation’s population. This is a telling statistic concerning the social woes of our nation’s minority groups. As for the type of crime, approximately half are in jail for violent crimes, including murder, rape, robbery, and assault. Most of the remaining prisoners are in jail for property crimes, which include burglary and various types of theft. Finally, there is a large and growing group that is incarcerated for drug offenses, whether manufacturing, selling, or using drugs.
Questions 56 to 58 are based on the passage you have just heard.
56. What aspect of crime does the professor mainly discuss?
57. According to the professor, what happened to the size of the prison population in America between 1990 and 2000?
58. Why does the professor discuss the Los Angeles County jail in detail?