Read the following passage, then choose the correct answer to questions.

PLANET EARTH  

      Planet earth is part of a system of planets and their moons, as well as numerous asteroids and comets which (21)_________around a huge star we call the Sun. The Sun itself (22)_________of gas. Nuclear reactions inside its core produce the heat and light that make life on the Earth (23)________. The Earth is the third of four small terrestrial (Earth like)planets. Further out in our Solar System are four huge gas planets, while distant Pluto, the smallest planet, is made (24)_________rock and ice. Our Earth is one of the ten planets that orbit the Sun- a huge, burning-hot star in the center of our Solar System. The Solar System and all the stars in the (25)________are part of our galaxy-the Milky Way, (26)______ contains as many as 200 billions stars. Beyond our galaxy are millions more galaxies. They all add together to make up the Universe.     The Sun itself (22)_________of gas.  

 Planet earth is part of a system of planets and their moons, as well as numerous asteroids and comets which (21)_________around a huge star we call the Sun.