Which of the Following Descriptions Constitutes a Controlled Experiment

Multiple Choice

Which of the following descriptions constitutes a controlled experiment?

A) using a microscope to observe organisms present in a sample of pond water and recording the number of each type of organism observed
B) setting up a bird feeder and compiling a list of all of the bird species seen at the feeder over a period of several weeks
C) growing one set of 10 seedlings under white light and one set of the same type of seedlings under red light and measuring their growth over a period of two weeks
D) growing one set of 10 seedlings under white light at 30°C and one set of a different type of seedlings under red light at 25°C and measuring their growth over a period of two weeks