How Did Astronomers Determine That the Planet Orbiting the Star

How did astronomers determine that the planet orbiting the star HD 209458 is a gas giant like Jupiter and not made mostly of rocks or metals?

A) they looked for light from the planet reflected into the spectrum of the star
B) they measured the mass of the planet, which immediately told them whether it was gas or rock
C) they watched the planet eclipse (cover) the light of the star, and thus were able to get the planet's size
D) they knew that any planet as close to its star as this one was could not be made of rock or metal
E) it took more than one of the above measurements to figure out this problem
Section 21.5: Exoplanets Everywhere - What We Are Learning