The large holes in Swiss cheese are due to the production of a large amount of $a$ by a bacterium $b$. Here $a$ and $b$ refer to:

• A
  $a – CO_2, b – \text{Penicillium roqueforti}$
• B
  $a – CO_2, b – \text{Propionibacterium sharmanii}$
• C
  $a – CO_2, b – \text{Penicillium notatum}$
• D
  $a – CO_2, b – \text{Saccharomyces cerevisiae}$