阅读理解   Stephanie Smith, a children's dance instructor, thought she had a stomach virus. The aches and cramping were tolerable that first day, and she finished her classes. Then her diarrhea turned bloody. Her kidneys shut down. Seizures knocked her unconscious. The convulsions grew so relentless that doctors had to put her in a coma for nine weeks. When she emerged, she could no longer walk. The affliction had ravaged her nervous system and left her paralyzed from the waist down.
    Ms. Smith, 22, was found to have a severe form of food-borne illness caused by E. coli, which Minnesota officials traced to the hamburger that her mother had grilled for their Sunday family party. In the simplest terms, she ran out of luck in a food-safety game of chance whose rules and risks are not widely known.
    Meat companies and grocers have been barred from selling ground beef tainted by the virulent strain of E. coli known as o157:H7 since 1994. Yet tens of thousands of people are still sickened annually by this pathogen with hamburger being the biggest culprit. Ground beef has been blamed for 16 outbreaks in the last three years alone. This summer, contamination led to the recall of beef from nearly 3,000 grocers in 41 states.
    Ms. Smith's reaction to the virulent strain of E. coli was extreme, but tracing the story of her burger shows that neither the system meant to make the meat safe, nor the meat itself, is what consumer have been led to believe.
    Ground beef is usually not simply a chunk of meat run through a grinder. Instead, a single portion of hamburger meat is often an amalgam of various grades of meat from different parts of cows and even from different slaughterhouses. This makes the costs 25% less than it would have for cuts of whole meat. These cuts of meat are particularly vulnerable to E. coli contamination, food experts and officials say. Despite this, there is no federal requirement for grinders to test their ingredients for the pathogen.
    Those ingredients include cuts from areas of the cow that are more likely to have had contact with feces, which carries E. coli, industry research shows. Yet most meat companies rely on their suppliers to check for the bacteria and do their own testing only after the ingredients are ground together.
    Unwritten agreements between some companies appear to stand in the way of ingredient testing. Many big slaughterhouses will sell only to grinders who agree not to test their shipments for E. coli for fear of a recall of ingredients they sold to others.
    'Ground beef is not a completely safe product,' said Dr. Jeffrey Bender, a food safety expert at the University of Minnesota who helped develop systems for tracing E. coli contamination. He said that while outbreaks had been on the decline, 'unfortunately it looks like we are going a bit in the opposite direction.'

单选题     What is the main idea of this passage?

单选题     We can learn from the passage that ______.

单选题     The reason behind the way ground beef is produced is ______.

单选题     We can infer that many big slaughterhouses ______.

单选题     As Bender implies at the end of the passage, ______.