What tiny, hair-like structures are used to help hard clams get food to its mouth?

Cilia are tiny, hair-like structures that play a crucial role in the feeding mechanism of hard clams and many other bivalves. These structures are covered in tiny hair-like projections that beat in a coordinated manner to create water currents. This movement helps to bring food particles, such as phytoplankton and detritus, towards the clam's mouth. The cilia function effectively in filtering and transporting food from the surrounding water into the clam's digestive system.

The other options, while they may seem plausible in describing structures involved in movement or feeding, do not accurately represent the specialized function that cilia perform in clams. Threads, feet, and strings do not have the same role or structure as cilia in the context of how hard clams capture and transport food. Cilia are specifically adapted for this filtering and feeding process, making them the correct choice for understanding how hard clams feed.