From which environment do hookworms and threadworms' larvae typically invade the skin?

Hookworms and threadworms' larvae typically invade the skin from soil. These parasites have a life cycle that includes a soil stage, where the larvae develop after hatching from eggs laid in areas contaminated with feces. The larvae can survive in moist soil and are often found in warm, humid environments that are conducive to their development.

When individuals come in contact with contaminated soil, typically barefoot walking, the larvae can penetrate human skin. This mode of transmission is crucial as it allows the parasites to enter the host and continue their life cycle inside the human body, leading to infections.

The other options don't provide the correct context for where these larvae thrive and initiate infection, as both freshwater environments and air are not typical habitats for these specific parasites. Sand, while it may harbor larvae, does not specifically represent the typical environment for hookworm transmission as clearly as soil does, which is the primary medium for their lifecycle. Therefore, the correct answer emphasizes the essential role of soil in the life cycle and transmission of hookworms and threadworms.