C is the answer
:) for your question (im writing extra cause answer is too small)
ADH acts on the "principal cells" to regulate facultative water reabsorption. ADH regulates facultative water reabsorption, through increasing the water permeability of the principal cells and reducing the urine produced .
Answer:
Pick Hydrogen, but read below.
Explanation:
The blacks are Carbon. They have 4 connections. The left Carbon has 3 whites connected to it. It is not totally an exclusive answer, but the whites could be anything in Column 17 (F, Cl, Br, I) or Hydrogen. Likely it is hydrogen.
