Silencing of a gene could be achieved through the use of
1. RNAi only
2. antisense RNA only
3. both RNAi and antisense RNA 
4. none of the above

(3) Gene silencing could be achieved through various routes including the use of RNAi, antisense RNA, ribozymes, etc. All these mechanisms interrupts or suppresses the expression of genes at transcriptional or translational level.