This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Microbiology
Which of the following is a bacteria taxonomically - 
Select and explain. 
A. Chlamydia, B. Rickettsia, C. Mycoplasma, D. All
D: All
Answer: D: All 
Explanation: Ans. is 'a' i.e., Chlamydia; 'b' i.e., Rickettsia; 'c' i.e., Mycoplasma Bacteria . Bacteria are single celled microorganisms which can exist either as independent (free living) organisms or as parasites (dependent on other organisms for life). . There are typically 40 million bacterial cells in a gram of soil and 1 million bacterial cells in a millilitre of fresh water; in all, there are approximately five nonillion (5 X IV ) bacteria on eah.