TOEFL Reading: ETS-TOEFL阅读机经 - 573AP9RN578GH4RKL$

According to paragraph 3, which of the following contributed to the use of epidemics? A. The development of more deadly forms of human disease B. The spread of ineffective treatments for infectious disease C. The spread of mosquito-borne diseases to other disease carriers D. The practice of exchanging goods between settled areas