Answer:
d. A water particle and an air particle
Explanation:
The force of gravity (F) between two objects of masses m1 and m2 and separated by a distance r is given as:
where G is the gravitational constant
This force is therefore, directly proportional to the masses and inversely related to the distance between them.
Based on the given options, since the masses of the water and air particles are very small (masses of earth, moon and sun is relatively huge), the gravitational force between them would be negligible and difficult to measure.