Which of the following formulas is correct?

• A
  Water potential $(\psi_{W}) = \text{Solute potential } (\psi_{S}) + \text{Pressure potential } (\psi_{P})$
• B
  Solute potential $(\psi_{S}) = \text{Water potential } (\psi_{W}) + \text{Pressure potential } (\psi_{P})$
• C
  Pressure potential $(\psi_{P}) = \text{Water potential } (\psi_{W}) + \text{Solute potential } (\psi_{S})$
• D
  Solute potential $(\psi_{S}) = \text{Water potential } (\psi_{W}) - \text{Pressure potential } (\psi_{P})$