Four test-tubes are each filled with starch solution and amylase. As shown in the table, they are kept in water-baths at different temperatures and at different pHs. All the other conditions remain unchanged. After 30 minutes, iodine solution is added to every test-tube. The mixture in three of the test-tubes becomes blue-black. In which test-tube do the contents remain yellow-brown? A: temperature $35^\circ\text{C}$, pH $2.5$ B: temperature $35^\circ\text{C}$, pH $6.9$ C: temperature $75^\circ\text{C}$, pH $2.5$ D: temperature $75^\circ\text{C}$, pH $6.9$
- ATest-tube A
- BTest-tube B
- CTest-tube C
- DTest-tube D