单选题 .  The accuracy of scientific observations and calculations is always at the mercy of the scientist's time-keeping methods. For this reason, scientists are interested in devices that give promise of more precise time-keeping.
    In their search for precision, scientists have turned to atomic clocks that depend on various vibrating atoms or molecules to supply their "ticking". This is possible because each kind of atom or molecule has its own characteristic rate of vibration. Atom in ammonia, for example, vibrates or "ticks" 24 billion times a second.
    One such atomic clock is so accurate that it will probably lose no more than a second in 3000 years. It will be of great importance in fields such as astronomical observation and long-range navigation. The heart of this Atomichron is a cesium (铯) atom that vibrates 9.2 billion times a second when heated to the temperature of boiling water.
    An atomic clock that operates with an ammonia molecule may be used to check the accuracy of predictions based on Einstein's relativity theories, according to which a clock in motion and a clock at rest should keep time differently. Placed in an orbiting satellite moving at a speed of 18,000 miles an hour, the clock could broadcast its time readings on a similar model. Whatever differences develop would be checked against the differences predicted.1.  The selection says that the accuracy of scientific observation depends on ______.