Answer the following question given this paragraph: 

Microfilaments , shown as (b) in Figure below , are made of two thin actin chains that are twisted around one another. Microfilaments are mostly concentrated just beneath the cell membrane, where they support the cell and help the cell keep its shape. Microfilaments form cytoplasmatic extentions, such as pseudopodia and microvilli , which allow certain cells to move. The actin of the microfilaments interacts with the protein myosin to cause contraction in muscle cells. Microfilaments are found in almost every cell, and are numerous in muscle cells and in cells that move by changing shape, such as phagocytes (white blood cells that search the body for bacteria and other invaders).


Q: Microfilaments are mostly concentrated just beneath what?


A:
the cell membrane