Studying one trait at a time ensures that multiple

are not changing simultaneously.

In an experiment usually, two groups are used  

1) Control Group  

2) Experimental Group

During the research, one trait variation at a time is studied in the experimental group in comparison to the control group.

For example:

If we want to study drought stress on plants we take plants group called control, which will be grown under normal conditions while we take other plant groups called the experimental group, which will be grown under all normal conditions except water. Water is given in the reduced form. Both these groups at the end of the experiment are compared with each other. Variation measured will be due to a single trait that is genes related to water stress.

the answer is d (apex) good luck : ))

step 1: nitrogen fixation- conversion of atmospheric no₂ into forms useful to plants by lightning, bacteria, and cyanobacteria.

step 2: nitrification-process of converting ammonia to nitrite or nitrate.

step 3: assimilation-plant roots absorb new form of nitrogen.

step 4: ammonification-conversion of organic nitrogen to ammonia by bacteria involved in decomposition.

step 5: denitrification-bacteria turns the decomposed matter back into n₂o, which is released into atmosphere; repeat cycle.