The curve given below shows enzymatic activity in relation to three conditions ($pH$,temperature and substrate concentration). What do the two axes ($x$ and $y$) represent?

• A
  $x$-axis: enzymatic activity,$y$-axis: $pH$
• B
  $x$-axis: temperature,$y$-axis: enzyme activity
• C
  $x$-axis: substrate concentration,$y$-axis: enzymatic activity
• D
  $x$-axis: enzymatic activity,$y$-axis: temperature