For the first one, the length of day is the right answer. Earth days are formed because the earth spins on its own axis, so one part of earth is getting sunlight and the other isn’t. If the earth rotates faster, the days will get shorter, and vice versa. For the second one, Venus is Earth’s sister planet. Both planets are around the same size, although Venus has a much higher temperature. For the last one, gravity keeps the planets in their orbits. If there was no gravity, the planets will be flung out into space and all the celestial bodies would crash into one another
