Isoflavonoids are known to a. act as phytoalexins
b. act as insecticides
c. cause infertility in mammals especially sheep
d all of the above

well, that's easy; the answer's c, "it reduces contamination of the water and substrate of the estuary."

a- multicellular

b-eukaryotic

c-no cell wall

d-heterotrophic

e-autotrophic

the answer is true when observing or determining population size, a direct observation is use, in which each organism is counted in a population or in an area. one benefit of this measure is that you get the exact population count but it will take a long time to  determine  the  size  of a  population.