参考答案：C

解析： 15-18
(Woman) Ladies and gentlemen, we all know that even with the advances of science and technology in our modern society it is very difficult for us to predict an earthquake, let alone prevent it. But people have long ago tried every possible means to record, and in some way, predict the occurrence of an earthquake. The world’s first instrument for recording earthquakes was invented in China in the second century AD. The instrument—today we would call it a seismoscope-was designed by a Chinese astronomer and geographer named Zhang Heng. It was a vessel-like device made of bronze and measured 6 feet in diameter. Inside the instrument there was a pendulum that would swing from the movement of a tremor that is too weak to be felt by a human being. When the pendulum swung, it would pull one of the attached bars. The bar would open the mouth of the dragon sculpture on the outside of the vessel and a bronze ball would drop and fall with a clang into the open mouth of a frog sculpture below. This seismoscope could not only record the movement of an earthquake, but also show from which direction the earthquake came.
In AD 138, Zhang Heng used his invention to announce that a major earthquake had struck 400 miles northwest of Luoyang, the Chinese capital. His report came long before messengers on horseback brought news of the earthquake to the capital city.
15.For what purpose was the vessel-like seismoscope invented