Three plants were grown to study the effects of nitrate and magnesium ion deficiency on their development. They were kept in the same conditions, except for the types of minerals supplied. Use
Plant A was provided with all essential minerals.
Plant B was given all minerals except nitrate ions. Plant C was given all minerals except magnesium ions. State three conditions, other than water and the concentration of mineral ions, that would need to be kept the same for all the plants, in order to make the investigation a fair test

Explanation:  

Controlled variable: Variables that are controlled and have no influence on the results of the experiment. These variables do not affect the change in the dependent variable values.  

These variables must be the same for all the subjects or groups (control and experimental). In the case of plants, controlled variables are probably those needed by the plants to correct development.  

All plants should be exposed to the same period of light-time and the same temperature. Also, the substrate or soil where the plants are growing should be the same.

you are certainly talking about this picture:

a: is the label of the white square, it is showing the  replication fork, formed by helicase (d) ==> so it is the site of dna separation and replication 

b: is a new dna brand that is following directly the replication fork, so it is continuous ==> built continuously from 5' to 3

c: dna polymerase cannot replicate dna on its own, it needs the of their proteins, and one of them plays the role of a clamp which fixes  the dna with the dna template and increases  the processivity of the enzyme. ==>   stops polymerase from falling off the  template.

d: is the enzyme responsible for forming the replication fork (helicase) ==> unwinds and separates dna into two strands

e: is the delayed strand, it is built  in the opposite sense of the helicase, so dna will be built in fragments ==> built  fragments from 5' to 3 joins

and finally f: is the enzyme that builds  the new dna strand (dna polymerase) ==>   adds nucleotides to make a new dna helix.

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answer: b

coz red blood cells marrow and circulates the body for 120 days.