单项选择题
A.①④ B.②③ C.①② D.③④
A.R型菌、S型菌、R型菌、R型菌 B.S型菌和R型菌、R型菌、R型菌、R型菌 C.R型菌、R型菌、R型菌、S型菌和R型菌 D.R型菌、S型菌和R型菌、R型菌、R型菌