In endocrine signaling,
a. the messenger molecules reach their target cells via passage through the bloodstream
b. the cell producing the messenger expresses receptors on its surface that can respond to that messenger
c. the messenger molecules are usually limited in their ability to travel around the body because they are inherently unstable or they are degraded by enzymes or they bind extracellular matrix
d. the messenger molecules travel only short distances through the extracellular space to cells that are in close proximity to the cell that is generating the message