Why do so many geneticists focus on studying yeast, fruit flies, nematodes, mice or the small plant arabidopsis?

They are all genetically tractable

Explanation:

Yeast, fruit flies, nematodes, mice or the small plant Arabidopsis have known genome i.e their entire genetic information is known to the scientific community, thus they are EASILY MODIFIED in genetic research.

Also, they are simple eukaryotes (having cells with defined nucleus) so they can explain the processes in the human eukaryotic cell during genetic research

Both reasons make them genetically tractable

the element that provides strength to the exoskeletons of clams and oysters is phosphorus.

this is so because calcium carbonate is the main component in the shells. calcium carbonate is strengthened by phosphorus.

explanation:

make   gliitsh plesea   !