Many rickettsia are PATHOGENS.
Pathogens are micro organism that cause diseases and rickettsia are majorly disease causing organisms. They are rod shaped bacteria  and most species are gram negative. They are natural parasite of some arthropods and they cause diseases in both humans and animals.
        
             
        
        
        
Answer:

Explanation:
We are asked to find how much of a 40 gram sample remains after 12 years. 
Iron-55 has a half-life of 3 years. Therefore, after 12 years, 4 half-lives have been completed. 
- 12 years/3 years = 4 half-lives
Every time a half-life is completed, half of the sample's mass decays. Remember we start with a 40 gram sample. 
- 1 half- life: 40 g / 2 = 20 g 
- 2 half-lives: 20 g / 2= 10 g 
- 3 half-lives: 10 g / 2 = 5 g
- 4 half-lives: 5 g / 2 = 2.5 g
There is also a formula that can be used to solve this problem. 

Where A₀ is the initial amount, t is the time, and hl is the half-life. 
We know 40 grams is the inital amount, 12 years is the time, and 3 years is the halflife. 




After 12 years, <u>2.5 grams </u> of Iron-55 will remain. 
 
        
             
        
        
        
This process is called Denitrification. The process is undertaken by the denitrifying bacteria, whose action is converting nitrates in the soil to free atmospheric nitrogen and therefore, depleting soil fertility and reducing agricultural productivity. Examples of denitrifying bacteria are thiobacillus denitrificans, micrococcus denitrificans among others.
        
             
        
        
        
Some of the important questions to be asked should be: Is the procedure of obtaining the results accurate? If not, by how much is its reliability? Are the results useful for further scientific research? What can you recommend to others who might want to make the same scientific conclusions?