On the outside of the plasma membrane, act as signals or markers to cells recognize one another.

The answer that fits this description is glycoproteins. glycoproteins are simple proteins which are also known as glycopeptides. they function as markers that aids cells to recognize one another. these proteins are located outside the plasma membrane.

The correct answer is c

Iwould say overuse ( but not 100% sure ) in the u.s. we tend to overuse water by leaving it running, throwing it away, taking long showers, etc.