D, Pennsylvania, Gettysburg, Address
        
             
        
        
        
Answer:
The scientist is studying oxygen which can also be found in protien. oxygen is the element which makes up to 65% of human body mass. It is also found in proteins inside the body.   
 
        
             
        
        
        
The student should chose a location within the office that has good ventilation and a workspace big enough to accommodate all his science materials. 
What is an experiment?
 An experiment can be defined as a procedure carried out under supervised conditions in order to discover an unknown effect, to test or refute a claim, or to illustrate a known law.
A science experiment should be carried out in a location within the office that has good ventilation and a workspace big enough to accommodate the required science materials. The location should be devoid of children, pets and other danger-posing objects.
In conclusion, experiments are carried out in suitable conditions and environment.
Learn more about experiments at: brainly.com/question/17274244
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Answer:
The fewer number of T cells a person has, the more chances to be affected by an opportunistic infection. When the number of T cells reaches 200 cells/µl, the person is at risk of being infected by other infections.  
Explanation: 
The whole immune system of a person who is infected with HIV infection seems to be severely affected.  When the person is not treated or the disease is in a very advanced stage, the depletion of the T- cells turns to be very sharped, especially CD4+ T cells. At this point, the immune system can not resist the attack of other microorganisms. The <em>lower is the number of CD4+ T Cells, the higher possibility the person has to be attacked by opportunistic infections. </em>
Classically, it has been suggested that opportunistic infections appear after the CD4 + T lymphocytes reach very low levels, such as 200 cells/µl. The number of circulating T cells can be used as an indicator and a measure of global "immune competence", and the previously mentioned amount of CD4 + T cells is an accepted universal reference used to predict the risk of having one of these opportunistic infections.