How Is C₄ Photosynthesis Able to Achieve Higher Concentrations of CO₂

How is C₄ photosynthesis able to achieve higher concentrations of CO₂ in the vicinity of rubisco than photosynthesis in C₃ plants can?

A) by separating carbon capture from carbon fixation
B) by suppressing photorespiration
C) by capturing HCO₃^- instead of CO₂
D) because the C4 cycle is faster than the Calvin cycle