Neptune's existence was predicted because a. Uranus did not seem to obey Newton's laws of motion. b. Uranus wobbled on its axis. c. Uranus became brighter and fainter in an unusual way. d. some of the solar nebula's mass was unaccounted for.

Newton's laws of motion are fundamental principles that describe the relationships between the motion of an object and the forces acting on it. There are three main laws:

1. **First Law (Inertia):** An object will remain at rest or in uniform motion unless acted upon by a force.
2. **Second Law (F=ma):** The force on an object is equal to its mass times its acceleration.
3. **Third Law (Action and Reaction):** For every action, there is an equal and opposite reaction.

These laws allow us to predict how objects move in space, including planets. When scientists noticed that Uranus did not move according to predictions made using Newton's laws, they suspected there was another force at play, hinting at the presence of another planet influencing its motion.

The irregularities in Uranus' orbit did not match Newton's expected trajectories. This discrepancy suggested that another massive object, Neptune, was exerting a gravitational pull on Uranus, forcing its observed paths to deviate from the predictions.

Uranus' orbit had peculiar anomalies that were difficult for astronomers to reconcile using existing models. These included:

- Discrepancies in Position: Uranus was not where astronomers expected it to be based on calculations.
- Orbital Wobble: Variations in its orbital path that couldn't be explained by known gravitational influences.

These anomalies led scientists to believe that an unknown gravitational force was affecting Uranus. By carefully studying these irregularities, astronomers could infer the presence and approximate location of another planet — Neptune. Thus, the discovery of Neptune resulted from the observed deviations in Uranus' orbit.

Gravitational effects are the influences that masses exert on one another due to their gravitational attraction. This is a core concept in understanding the dynamics of celestial bodies.

In the case of Neptune's prediction, scientists used the observed gravitational effects on Uranus to deduce the existence of another planet. Here's how:

1. **Unexplained Movements:** Uranus displayed motions that couldn't be explained by the known planets' gravitational forces.
2. **Calculations:** Mathematicians such as Urbain Le Verrier and John Couch Adams calculated the position and mass of the unseen body (Neptune) influencing Uranus.
3. **Validation:** When telescopes were aimed at the calculated position, Neptune was indeed discovered.

Thus, understanding gravitational effects allowed scientists to predict and eventually confirm Neptune's existence, demonstrating the power of Newton's laws and the precision of astronomical calculations.