The solutions, which have both hydrogen and hydroxide ions in equal number are neutral solution.
In a solution, if the hydrogen and the hydroxide ion are in equal number, they will mask each other's effect. So, all over, the effect of the hydrogen ion or hydroxide ion would be absent. So, the solution would indicate a pH of value 7.
Hence, the given blank can be filled with neutral.
Answer:The NASA astronauts from the Apollo missions that landed on the moon experienced 1/6 of the gravitational force that they usually experience on Earth. Provide an explanation for the astronaut's reduced gravitational force.