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 -a) Chlamydiab) Rickettsiac) Mycoplasmad) Prione) Bacteriophage 
Select and explain. 
A. ab, B. abc, C. acd, D. bcd
B: abc
Answer: B: abc 
Explanation: 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 1030 ) bacteria on earth.