19. Why are the inner planets rocky while outer planets are gaseous?  
A) Higher temperatures near the Sun allowed only heavy rocky materials to condense.  
B) Gravitational pull of the Sun pulled all gases inward, leaving rocks outside.  
C) Outer planets formed later from comet debris.  
D) Solar winds preferentially removed rocks from inner orbits.  

20. The Sun's energy comes mainly from:  
A) Chemical burning of hydrogen like a giant fire.  
B) Nuclear fusion of hydrogen into helium in its core.  
C) Gravitational contraction releasing heat over time.  
D) Radioactive decay of heavy elements in its interior.  

21. A key difference between terrestrial planets and gas giants is:  
A) Terrestrial planets have solid surfaces and fewer moons; gas giants have no solid surface and many moons.  
B) Gas giants are smaller with thin atmospheres; terrestrial planets are larger with thick rings.  
C) Terrestrial planets have rings; gas giants have solid rocky cores visible from space.  
D) Gas giants orbit closer to the Sun with higher densities.  

22. The Great Red Spot is a feature of:  
A) Saturn's ring system.  
B) Jupiter's atmosphere, a persistent storm.  
C) Mars' polar ice caps.  
D) Neptune's dark spots.  

23. What causes a comet to develop a tail?  
A) Gravitational pull from nearby planets stretching the object.  
B) Solar radiation and wind vaporizing ices as it approaches the Sun.  
C) Collision with asteroids releasing dust particles.  
D) Magnetic interactions with the solar wind creating auroras.  

24. The heliocentric model was supported by observations of:  
A) Planets showing retrograde motion explained by relative orbital speeds.  
B) Fixed stars appearing to move in perfect circles around Earth.  
C) The Moon always facing the same side toward the Sun.  
D) All planets maintaining equal distances from Earth.  

25. Which planet has the lowest density in the solar system?  
A) Jupiter, due to its massive gaseous composition.  
B) Saturn, which could theoretically float in water.  
C) Earth, because of its balanced rock and water mix.  
D) Mercury, due to its small size and metallic core.  

26. The solar system includes eight planets because:  
A) Pluto and other dwarf planets do not meet the criteria of clearing their orbital paths.  
B) Only eight objects are large enough to be seen with the naked eye.  
C) Historical classification limited it to visible wandering stars.  
D) Gravitational stability allows only eight stable orbits.  

27. Planets orbit the Sun in paths that are best described as:  
A) Perfect circles as proposed in early models.  
B) Ellipses with the Sun at one focus.  
C) Random trajectories influenced only by nearby moons.  
D) Spirals gradually moving inward over time.  

28. The main difference between meteors and meteorites is:  
A) Meteors burn up in the atmosphere; meteorites survive and land on the surface.  
B) Meteors originate from comets; meteorites from asteroids only.  
C) Meteorites are larger and always create craters.  
D) Meteors are artificial objects while meteorites are natural.  

29. The formation of the solar system explains why:  
A) All planets have identical compositions regardless of distance from the Sun.  
B) Lighter gases are more common in outer planets while inner planets are rocky.  
C) The Sun revolves around Earth in a geocentric pattern.  
D) Comets only appear in the inner solar system.  

30. A model of the solar system should demonstrate that:  
A) Planets revolve around the Sun at different speeds and distances, with the Sun at the center dominating by mass.  
B) All planets move at the same speed in circular orbits around Earth.  
C) The Sun is just one small body among many equal planets.  
D) Moons and asteroids orbit independently without gravitational influence from the Sun.