Frederick Griffith conducted experiments testing the virulence of two strains of Streptococcus (R-strain and Sstrain). His experiment had four specific "treatments" to it. Treatment 1, he injected mice with the R-strain and the mice lived. Treatment 2, he injected mice with the S-strain and the mice died. Treatment 3, he injected mice with heat-killed S-strain and the mice lived. Treatment 4, he injected mice with heat-killed S-strain mixed with R-strain and the mice died. Which treatment in his experiment is considered the experimental treatment?