Answer:
There was no control group.
Step-by-step explanation
In this problem, there are two temperatures given (20 degrees Celsius and 40 degrees Celsius). The problem then states that Group A and Group B are going to be set to their separate temperatures. The problem wants you to think that the constant here is Group A, because they state that the temperature is being maintained at room temp. Because we don't know the temperature of the room or fish tanks beforehand, we have no idea if the tanks were different temperatures then they are now. Because we have no data to compare to the tanks, we have no control group in this problem. Be careful to read through these problems and don't let the wording of the problem confuse you. By using words such as 'maintained' and 'raised to', the problem makes you think that Group A is the control. Just because Group A is being maintained at room temperature doesn't mean it wasn't room temperature before the experiment was held. Just read through the problem and you'll be fine :)