Which of the following statements is correct about the given diagram of the cardiac cycle?

• A
  '$a$' represents atrial systole, during which $25\%$ of blood moves from atria to ventricles actively.
• B
  $(a \rightarrow b \rightarrow c)$ represents $(\text{Atrial systole} \rightarrow \text{Joint diastole} \rightarrow \text{Ventricular systole})$.
• C
  '$c$' and '$d$' together are called joint diastole, during which approximately $75\%$ of blood moves from atria to ventricles passively.
• D
  '$b$' represents the phase of ventricular systole.