Why does a neutral solution have a of 7.0?

in a neutral solution, the [h3o+]=1×10^−7 at 25∘c. ph=−log[1×10^−7]=7.0.
in a neutral solution, the [h3o+]=1×10^7 at 25∘c. ph=log[1×10^7]=7.0.
in a neutral solution, the [h3o+]=1×10^14 at 25∘c. ph=0.5log[1×10^14]=7.0.
in a neutral solution, the [h3o+]=1×10^−14 at 25∘c. ph=−0.5log[1×10^−14]=7.0.