TOEFL Reading: ETS-TOEFL阅读机经 - 8L9E4LFT54FUKKRLL$

Why does the author provide the information that "The R strain lacks this coating" A. To provide an example of variations within strains of pneumococcus bacteria B. To explain why the R strain is not able to cause disease C. To suggest that the R strain has other ways to defend itself from immune defense mechanisms D. To explain why mice became diseased when injected with the R strain