Factors that affect enzyme action — practice question

A group of students studied how pH affected the speed of starch digestion by amylase at $20\,^{\circ}\mathrm{C}$. Into a test-tube, they placed $2\,\mathrm{cm}^3$ of a 1% amylase solution together with $1\,\mathrm{cm}^3$ of pH 4 buffer. They then added $2\,\mathrm{cm}^3$ of a starch solution, which contained $0.02\,\mathrm{g}$ of starch, to the test-tube. The time for the starch to be digested (to disappear) was recorded. These steps were then carried out again using pH 5, pH 6, pH 7 and pH 8 buffers. The results are shown in the table: pH 4 → 4.0 min, pH 5 → 1.5 min, pH 6 → 1.0 min, pH 7 → 2.5 min, pH 8 → 6.0 min. Which graph matches these data?