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Mayra should be checked regularly for heart-related problems.
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I think she should try the experiment one set with fertilizer, and one without. The negative control group would be the set of rose bushes grown without the fertilizer, but under the exact same conditions as the experimental group. Then, if a difference is seen you can be sure it's accurate, since you tested with and without fertilizer under the same conditions. Sorry if i am wrong, I am not completly sure about this one. Again I might be wrong so sorry in advance.
You would use a beaker because it measures liquids and solids by volume.
To get a more accurate measurement, you should weigh the liquids. That process of measurement would give you the greatest accuracy.