TOEFL Reading: ETS-TOEFL阅读机经 - 19OKR71R9A6BHX6QL$

According to paragraph 1, Griffith experimented with strains of the pneumococcus bacteria because he wanted to discover which of the following A. A strain of bacteria that could be used to develop a vaccine B. How bacterial strains developed under laboratory conditions C. Why the strains of bacteria differed in appearance D. Which bacterial strains were most infectious in humans