Blood is drained from a convenient $P$ of a patient and is pumped into a dialyzing unit after adding $Q$. After purification,$R$ is added to the blood,and it is returned to the body through $S$.
$\quad \quad P\quad \quad Q\quad \quad R\quad \quad S$

• A
  Vein $\quad$ Heparin $\quad$ Anti-heparin $\quad$ Artery
• B
  Artery $\quad$ Heparin $\quad$ Anti-heparin $\quad$ Vein
• C
  Artery $\quad$ Anti-heparin $\quad$ Heparin $\quad$ Vein
• D
  Vein $\quad$ Anti-heparin $\quad$ Heparin $\quad$ Artery