A student carried out an investigation into how a high temperature affects the formation of nitrate ions in soil. Two soil samples were used. One of the samples was heated to $100^\circ\text{C}$. All nitrate ions were then removed entirely from both samples of soil. Next, ammonium ions were added to each soil sample. After two weeks, both samples of soil were tested to find out whether nitrate ions were present. The results are shown. Which statement explains the results?
- AHeating the soil broke down the nitrate ions.
- BHeating the soil increased the activity of denitrifying bacteria.
- CHeating the soil killed nitrifying bacteria.
- DHeating the soil killed nitrogen-fixing bacteria.