In RNAi, the genes are silenced using

In RNAi technology, the target nematode genes are silenced using dsRNA.
Eukaryotic cells have iRNA based defense mechanism meant to destroy their own mRNA after translation or a dsRNA formed because of viral infection. Plant cells have sense and anti-sense RNA for its own genes but not for nematode genes. When a nematode infests a tobacco plant by inserting its genes into the host plant and making mRNA (sense), this nematode mRNA is continuously translated to help continue the infection of plant roots. A transgenic tobacco plant is the one into which a nematode gene (critical for its survival) is inserted in a reverse orientation. The mRNA synthesized by this reverse oriented gene is in antisense orientation, and falls complementary to the nematode
mRNA. When a nematode infects such transgenic tobacco, it inserts its genes and initiates transcription, and nematode mRNA (sense strand) is formed. At the same time, the cell also expresses the reverse oriented nematode gene which forms an mRNA (antisense strand). Since sense and anti-sense strands complement each other, they form a dsRNA. Formation of a dsRNA activates a dicer-based defense mechanism in eukaryotic cells. The dicer proteins dice the dsRNA. Therefore, nematode mRNA is continuously destroyed by this mechanism, so nematode cannot survive in a transgenic tobacco plant.
(2), (3), (4) ssDNA, ssRNA, dsDNA are not involved in gene silencing.