How many variables should be manipulated in a correctly performed scientific experiment?
a) one
b) three
c) two
d) zero

The correct option is A) one

Explanation:

Through scientific experiments, we can test the effect of a certain variable. In a scientific experiment, a dependent variable can be described as the variable which is changed by the scientists so that its effect can be studied on the dependent variable. It is ideal if only one variable is changed at a time. If more than one variable is changed, then it will be difficult to understand which variable brought about the change which is being seen or depicted. Hence, only one variable should be changed while performing a scientific experiment.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

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.