Nitrogen fixation is carried out primarily by humans by...

A. Humans.
B. Plants.
C. Bacteria.
D. Consumers.

Since this is not my area of expertise, I searched Google under the key words "Nitrogen fixation" to get these possible sources:

http://en.wikipedia.org/wiki/Nitrogen_fixation
(Broken Link Removed)

In the future, you can find the information you desire more quickly, if you use appropriate key words to do your own search. Also see http://hanlib.sou.edu/searchtools/.

The process of nitrogen fixation involves converting atmospheric nitrogen gas (N2) into a usable form, such as ammonia (NH3) or nitrate (NO3-), which can then be taken up by plants. While humans do play a role in nitrogen fixation, it is primarily carried out by certain types of bacteria, specifically, nitrogen-fixing bacteria.

To get the answer to the question, we need to understand the process of nitrogen fixation and the organisms involved. Nitrogen fixation occurs in nature through two primary mechanisms: biological nitrogen fixation and abiotic nitrogen fixation.

Biological nitrogen fixation is the conversion of atmospheric nitrogen gas (N2) into a biologically usable form by living organisms. In this process, certain bacteria, called nitrogen-fixing bacteria, have the ability to convert nitrogen gas into ammonia (NH3). They possess an enzyme called nitrogenase, which enables this conversion. These bacteria are found in the roots of certain plants, such as legumes (e.g., beans, peas, clover), where they form a symbiotic relationship with the plant. In this symbiosis, the bacteria provide ammonia to the plant, which helps in the plant's nitrogen nutrition, while the plant provides carbohydrates as an energy source for the bacteria.

Besides biological nitrogen fixation, there is also abiotic nitrogen fixation, which occurs through non-living processes. This includes lightning strikes, which can convert nitrogen gas into nitrate through high-energy reactions in the atmosphere.

Given this information, the correct answer to the question "Nitrogen fixation is carried out primarily by humans by..." would be C. Bacteria. Humans do contribute to nitrogen fixation processes through various agricultural practices, such as planting legume crops or applying nitrogen-fixing bacteria as biofertilizers. However, the primary role in carrying out nitrogen fixation lies with bacteria.