Concept:Viral diseases of crops cannot be cured by any chemical, so prevention is the only effective way to control them.
Explanation:Viruses live inside plant cells, and no chemical spray can kill them without damaging the plant.
Spraying insecticides may kill the insect vectors, but it cannot cure plants that are already infected with the virus.
Vector control only reduces the spread of the disease, but it does not offer complete protection.
Sterilizing farm equipment helps prevent the transfer of viruses from infected plants to healthy ones, but it is only a minor control measure.
Planting resistant varieties prevents the virus from establishing itself in the crop right from the start.
This method is more sustainable, cost-effective, and highly reliable because the plants naturally resist the virus.
Therefore, resistant varieties give the best long-term control of viral diseases in crops.
Answer:B. Planting resistant varieties