All bacteria are classified as
a. eukaryotes.
b. prokaryotes.
c. archaea.
d. protista.

The best answer is B - Prokaryotes.

Bacteria do not have a membrane bound nucleus and other internal structures. They have therefore been placed in the class of prokaryotes.

The dominant living creatures on earth are the prokaryotes. They have been able to adapt to nearly all ecological habitats that are available.

Some types of bacteria can cause disease in human beings, animals and even plants. However most are quite harmless.

Other bacteria serve as important ecological agents in that they fix nitrogen in the soil and degrade  cellulose. Soil would not be fertile without bacteria. 

i'd say that the moon rock is about 2.2 billion years old.

b. the carbon cycle ensures living things have a source of hydrogen