In city apartments and dwellings, the presence of cockroaches, fleas, ants, mosquitoes or houseflies is guaranteed to elicit the spraying of insecticides. Mice and rats are poisoned or trapped, while the gardener wages a never-ending struggle with ragweed, dandelions, slugs and root-rot. We have a modern arsenal of chemical weapons to fight off theseinvadersand we use them lavishly.The following are invaders except________.

A. cockroaches
B. mice and rats
C. dandelions
D. gardeners