TOEFL Reading: ETS-TOEFL阅读机经 - 6UQ10TM3F83I4163L$

Which of the following can be inferred from paragraph 6 about underground sewers? A. They became common in most of Western Europe in the 1830s. B. They helped reduce deaths caused by disease in cities. C. They led to the discovery that disease could be caused by germs. D. They encouraged people to leave rural areas and move to the cities.