Câu hỏi:

Read the following passage and mark the letter A, B, C or D on you answer sheet to indicate the correct answer to each of the questions. 

SCIENTIFIC DISCOVERIES

      A satellite is best understood as a projectile, or an object that has only one force acting on gravity. Technically speaking, anything that crosses the Karman Line at an altitude of kilometers (62 miles) is considered in space. However, a satellite needs to be going fast — at least 8 km (5 miles) a second - to stop from falling back down to Earth immediately. 

      If a satellite is traveling fast enough, it will perpetually "fall" toward Earth, but the Earth's curvature means that the satellite will fall around our planet instead of crashing back on the surface. Satellites that travel closer to Earth are at risk of falling because the drag of atmospheric molecules will slow the satellites down. Those that orbit farther away from Earth have fewer molecules to contend with. 

      There are several accepted "zones" of orbits around the Earth. One is called low-Earth-orbit, which extends from about 160 to 2,000 km (about 100 to 1,250 miles). This is the zone where the ISS orbits and where the space shuttle used to do its work. In fact, all human missions except for the Apollo flights to the moon took place in this zone. Most satellites also work in this zone. 

      Geostationary or geosynchronous orbit is the best spot for communications satellites to use, however. This is a zone above Earth's equator at an altitude of 35,786 km (22,236 mi). At this altitude, the rate of "fall" around the Earth is about the same as Earth's rotation, which allows the satellite to stay above the same spot on Earth almost constantly. The satellite thus keeps a perpetual connection with a fixed antenna on the ground, allowing for reliable communications. When geostationary satellites reach the end of their life, protocol dictates they're moved out of the way for a new satellite to take their place. That's because there is only so much room, or so many "slots" in that orbit, to allow the satellites to operate without interference. 

      While some satellites are best used around the equator, others are better suited to more polar orbits - those that circle the Earth from pole to pole so that their coverage zones include the north and south poles. Examples of polar-orbiting satellites include weather satellites and reconnaissance satellites. 

(Source: https://www.space.com/24839-satellites.html)

Which of the following is mentioned in paragraph 4 as an element that makes a satellite stay above the same spot on Earth?