Epidemiologic studies that examine the role of a suspected factor in the etiology of a disease may be observational or experimental. The most important difference between experimental and observational studies is that in experimental studies:

a. The study and control groups are equal in size.
b. The study is prospective.
c. The study and control groups are always comparable with respect to all factors other than the exposure.
d. The investigator determines who shall be exposed to the suspected factor and who shall not.
e. Controls are used.

the correct option is D (The investigator determines who shall be exposed to the suspected factor and who shall not.)

Explanation:

Epidemiology is the branch of medicine that deals with the study of diseases in a defined population to obtain evidence that will aid in the prevention, treatment and control. It is divided into 2 categories:

- experimental studies and

- Observational studies.

In Observational studies, the epidemiologist collects data of interest based on what is seen or observed from the exposure or disease status of each study participants. While in experimental studies, the investigator determines through a controlled process the exposure for each individual and then track over time to detect the effects of the exposure. Therefore in experimental studies,the investigator determines who shall be exposed to the suspected factor and who shall not. I hope this helps, thanks!

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.

credit: gradman555p98x9p

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explanation:

the universal gravitation equation. since the gravitational force is directly proportional to the mass of both interacting objects, more massive objects will attract each other with a greater gravitational force. if the mass of one of the objects is doubled, then the force of gravity between them is doubled.