Step-by-step explanation:
Why:
Because if the temperature is held constant during an experiment, it is controlled. Some other examples of controlled variables could be the amount of light or constant humidity or duration of an experiment etc.
How:
The light intensity (how close the light is). Dependant variable - the number of oxygen bubbles given off (the rate of photosynthesis). Controlled variables - the size of the pondweed, the volume of water used and its temperature.