1. A. The relative motion between Earth and the Sun
The Earth is constantly moving, be it around its own axis, or around the Sun. As it moves around the Sun over the course of one year, the Earth changes its relative position toward the constellations. This results in different positions of the constellations in different periods of the year, when it comes to viewing them from the Earth of course. It looks for the ordinary observer that the constellations are making lot of movement throughout the year and change their positions, and they do move, just not in that manner, but in fact it is the relative motion between the Earth and the Sun that gives that effect on the night sky.
2. A. Interstellar medium
The galaxies are composed of multiple basic things, with the stars and the interstellar medium being the dominant ones. Apart from these two there are also gases and dust. The galaxies are massive, and even the smallest of them are so big that we can not even imagine in our minds as we don't have a scale in our minds for something so large. They have trillions of stars in them, and the majority of the space between the stars is filled with interstellar medium.
3. B. Expand
The Big Bang theory is the most widely accepted theory about the formation of the universe, even though it doesn't really has proper explanations for certain things. This theory suggests that everything managed to get dragged into a single miniature spot and once the gravity became way to big, an explosion occurred. This explosion was so big, and everything moved so quickly, that in literary thousands of parts of a second millions of light years of space were filled in. Everything was practically expanding from the spot of explosion, expanding outwards, giving the basis for the formation of everything we see in the universe. The process of expanding is still continuing, as we can see with the quasars, and it is expected to do so in the future.
4. C. Low-mass main sequence
From what has been observed from the space until now, it seems that the low-mass main sequence stars are the once that dominate the universe by far, at least in numbers. These stars are also called red dwarfs. They are small stars, having reddish color, and their mass is also relatively low compared to the bigger stars. It is estimated that around three quarters of the stars are actually of this type. Unfortunately, because they are small and their low luminosity, individual stars of this type can not be seen from the Earth with naked eye.
5. A. Rocky
The terrestrial planets are planets that are small in size. They are closer to the Sun, thus they are warmer because of it. Unlike their Jovian counterparts, the terrestrial planets tend to be denser, but also what is a striking distinction is that they are rocky. These planets have rocky surfaces, thus their surfaces are solid, covered with numerous types of geologic features, while the Jovian do not have rocky surface, but instead it is gaseous or icy. The fact that these planets are rocky, also gave big advantage to them when it came to providing conditions for life, and the Earth was the lucky one that had all other things in place and boomed with life.
6. B. The Moon's gravity pulls on Earth's oceans, causing tides
The Moon is very close to the Earth, and it is actually a very large moon for the size of the Earth. It has its own gravity of course, and the sheer proximity means that it will influence on something. Since the gravitational pull of the Earth is stronger, the gravitational pull of the Moon can not do something spectacular, but at least it manages to pull the ocean waters. As it pulls the ocean waters, the Moon's gravity actually cause the high tides and low tides.
