Answer the following question given this paragraph: 

Each endocrine hormone affects only certain cells, called target cells. A target cell has proteins on its surface to which a given hormone can bind. Most endocrine hormones are controlled by negative feedback loops. Negative feedback occurs when low levels of a hormone feed back to increase its secretion—and vice versa.


Q: Each endocrine hormone affects only certain "target" cells, which have molecules of what on the surface that enable binding of a given hormone?

 Choices:

- lipids

- rna

- gas

- protein

A:
protein
