How did Copernicus figure out which planets were closer to the sun and which were further?
A) By observing the planets' brightness at night
B) By measuring the planets' apparent sizes in the sky
C) By studying the planets' retrograde motion
D) By determining the planets' orbital periods around the sun
Choose one of the following options:
a) A and B
b) B and C
c) C and D
d) A and D