In Fred Griffith's Experiments, Harmless R Strain Pneumococcus Became Lethal

In Fred Griffith's experiments, harmless R strain pneumococcus became lethal S strain pneumococcus as the result of which of the following?
1) horizontal gene transfer
2) transduction
3) conjugation
4) transformation
5) genetic recombination

A) 2 only
B) 4 only
C) 2 and 5
D) 1, 3, and 5
E) 1, 4, and 5