Why does the oxygen atom in a water molecule have a negtive change

Oxygen has a negative ion.

Explanation: In a water molecule, the oxygen atom and hydrogen atoms share electrons in covalent bonds, but the sharing is not equal. The unequal sharing of electrons gives the water molecule a slight negative charge near its oxygen atom and a slight positive charge near its hydrogen atoms.

b. the carbon cycle ensures living things have a source of hydrogen

i'm thinking (a) to keep sand in place; ////////////

explanation: a sand fence or sandbreak, like a snow fence, is used to force windblown, drifting sand to accumulate in a desired place. sand fences are employed to control erosion, sand dune stabilization, keep sand off roadways, and to recruit new material in desert areas.