Commensalism refers to relationship wherein one organism is benefitted while or remains unaffected. Mutualism is association of organisms wherein both are benefitted. Ergets are small birds that sit on back of cow and mostly feed on grasshoppers that are stirred when herd move through field/cropland. Ergets derive nutritional benefit from this association while cow remain unaffected, making it an example of commensalism. Or examples represent mutualism wherein both partners are benefitted. Thus, correct answer is option A.

The warm-blooded animals are called homeorms. They have ability to maintain ir body temperature at a constant level despite change in temperature of surrounding environment. Examples are mammals, birds.

(a) Birth rate or natality is number of live births per 1000 people of population per year
(b) Death rate or Mortality is number of deaths per 1000 people per year.
(c) Population density is number of individuals per square kilometre at any given time.
(d) Exhaustible resources are those resources which are present in limited quantity and can be completely used up by human activities are called exhaustible resources.
(e) Growth rate of population is difference between birth rate and death rate. As long as birth rate exceeds death rate, population grows. If birth rate is lower than death rate, population declines.