People can use to discuss how and why atoms form bonds

People can use models to discuss how and why atoms form bonds.

In chemistry, the ball-and-stick model is a molecular model of a chemical substance which is to display both the three-dimensional position of the atoms and the bonds between them.

The atoms are represented by spheres, connected by rods which represent the bonds. Every ball represent one atom in one molecule and they have different colors for different atoms (for example, carbon is black, hydrogen is white, nitrogen is blue).

There also other models: skeletal models, polyhedral models, composite models, computer-based models.

b i think it’s a water wheel turning in a stream

the answer would be a