Carlos is trying to determine how music affects the growth of bean plants. He hypothesizes that classical music will cause a plant to grow at a higher rate than no music or rap music. He grows nine bean plants in similar lighting, temperature, and soil conditions. Three are exposed to classical music 24 hours a day, three are exposed to rap music 24 hours a day, and three are grown in complete silence. Carlos measures the growth of the plants every day and finds that the plants all seem to have the same rate of growth. What is missing from Carlos's scientific investigation?
A: A specific hypothesis predicting a relationship between independent and dependent variables
B: Controlled variables to ensure only the independent variable is being changed
C: Experimental data collected regularly that can be recorded and analyzed
D: Research on what other scientists have observed and investigated on this topic