a substance that increases the rate of a chemical reaction without itself undergoing any permanent chemical change.
- a person or thing that precipitates an event.
I think it might just might be e
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.
Humans.
Humans have mass and take up space. :)
Hope this helps!