Phosphorus is:
1 found free in the atmosphere
2 returned to the soil by rhizobium
3 added to the soil through weathering
4 usually very abundant in an ecosystem

added to the soil through weathering  

Explanation:

1) found free in the atmosphere : in it is found in the form of phosphates etc.

2 returned to the soil by rhizobium : Rhizobium is a bacteria which is involved in nitrogen fixation.

3 added to the soil through weathering  : It is added to soil again by weathering and is absorbed by plants in the form of phosphates.

4 usually very abundant in an ecosystem: it is found in earth nearly 0.1% by weight.

the grams   of   barium that   are in   2.3 x 10^23   atoms of barium   is   52.4 g (answer c)

    calculation

mass = moles   of ba   x molar   mass of ba

  molar mass of ba from periodic table=   137.3 g/mol

  use the   avogadro's law   constant   to   calculate the moles of barium

  that   is     1 mole =   6.02   x 10^23 atoms

                    ? moles = 2.3 x 10^23   atoms

by cross   multiplication

=(1 mole   x 2.3x 10^23 atoms)   / 6.02   x10^23 atoms)=   0.382   moles

mass   is therefore = 137.3   g/mol x 0.382 mol = 52.4 ( answer c)

explanation:

one negative thing it that it can kill water animals. another negative thing is it can cause the destruction of ecosystem. one way water pollution can be reduced is to conserve energy.