Jenny decides to examine the effect of temperature on the tuning of a trumpet. She tunes 5 trumpets and then leaves them in their cases during the day. She places 4 trumpets in places of varying temperatures (some hot places and some cold places) and leaves 1 trumpet in the temperature-controlled room she tuned it in. After 8 hours, she takes all 5 trumpets out of their cases and records how the tuning of the trumpets has changed. She also makes sure to hold all experimental factors constant except for the temperature. Which statement describes what Jenny did to prepare the control group in her experiment?

Jenny held all experimental factors constant except the temperature
Jenny kept one trumpet in a temperature controlled room.
Jenny placed four trumpets in environments with different temperatures
Jenny measured the tuning of all trumpets after 8 hours