Answer:
B. Insects with a resistance gene survived the first cyanide sprayings and passed the gene to their offspring
Step-by-step explanation:
So, just by chance, one or more bugs already had some genes that provided them resistance to fight the cyanide. These bugs survived and the rest died. So since only the resistance bugs survived, they transferred their genes onto their kids and now kids are also immune. Now the cycle continues over generation until everyone is pretty much immune to the sprays.
This is exactly why we are currently facing and will face massive deaths due to bacterial resistance to the antibiotics we use.