In a water culture experiment, the culture solution in a transparent beaker turned green. What precaution
should have been taken to prevent the culture solution from turning green?

A. Changing the culture solution regularly
B. Increasing the number of times the culture is aerated
C. Covering the transparent beaker with black cloth  
D. Adding warm alcohol to bleach the green colour
E. Sterilizing the beaker in a hot oven before use

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Correct Answer: C