To determine which two planets should be closest in diameter on the scale model, we can look at the given diameters of the planets and find the pair that has the smallest difference.
Here are the diameters for reference:
- Mercury: 4,800 km
- Venus: 12,100 km
- Earth: 12,800 km
- Mars: 6,700 km
- Jupiter: 143,000 km
- Saturn: 120,000 km
- Uranus: 51,100 km
- Neptune: 50,000 km
Now, let's compare the diameters of the planets pairwise to find the closest two:
- Mercury (4,800) and Neptune (50,000): Difference = 45,200
- Mercury (4,800) and Uranus (51,100): Difference = 46,300
- Earth (12,800) and Venus (12,100): Difference = 700
- Mars (6,700) and Jupiter (143,000): Difference = 136,300
- Neptune (50,000) and Uranus (51,100): Difference = 1,100
- Saturn (120,000) and Jupiter (143,000): Difference = 23,000
The smallest difference in diameter is between Earth and Venus (700 km) and also between Neptune and Uranus (1,100 km).
Based on the available responses, the closest pair in diameter is Earth and Venus.
So the answer is Earth and Venus.