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a scientist attempts to determine the effect of varying concentrations of ammonia on a bacterial culture. several cultures of the
same species of bacteria were grown in culture tubes, each with a different concentration of ammonia. the cultures were
incubated (kept in a warm, safe environment) and after 24 hours the cultures were examined for bacterial growth.
to improve this experimental design, the scientist should have included a control group which
a
contained no ammonia
b
contained no bacteria
с
was not incubated
d
was incubated at room temperature
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