A. The planet, Mercury
B. Observation of planets
C. The Earth's moon
D. Astronomers' difficulties
A. Because it has an atmosphere which obscures observation
B. Because it is too close to the sun
C. Because it is too small
D. Because it is difficult for space probes to get close to
A. The valleys are filled with volcanic rocks
B. There are no volcanoes
C. The valleys are uncratered
D. There are hilly regions
A. Show, the volcanoes on Mercury
B. Show the meteorites which hit Mercury
C. Show what caused Mercury's valleys
D. Show more of the planet, Mercury
A. Better and more space telescopes
B. More hypotheses
C. Larger space telescopes
D. More ways to prove hypotheses
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