A、Horsemen. B、Brass doors. C、Drops of water. D、Metal balls. D原文提到，“每隔一个小时就打开一扇门，适当的金属球数落入一个薄黄铜盘报时”，可知正确选项是D(金属球)。

The water clock was an ancient clock. It could be used on cloudy days, at night, and indoors. Such clocks were probably first used in Egypt [16]about 2,000 B.C. They were also used for many years in Arabia, India, China, and all of Europe.
    The simplest water clock needed just two parts—a small bowl and a large bowl. The small bowl had a tiny hole in the bottom; the large bowl was divided into equal parts, and marked by lines. [17]Water was poured into the small bowl, and it fell slowly down into the large bowl in tiny drops. Usually it took an hour for the water to rise from one line to the next in the large bowl.
    A beautiful water clock was made in Persia over a thousand years ago. It was presented as a gift to the Emperor of France. This water clock could be heard as well as seen. The dial of the clock was made up of twelve doors, each representing an hour. [18]Every hour a door opened, and the proper number of metal balls dropped onto a thin brass plate to strike the hour. At twelve o’clock, twelve tiny horsemen came out and shut all the doors. This was probably one of the first striking clocks.
16. When was the first water clock probably made?
17. What do we know about the structure of the simplest water clock?
18. What did the striking water clock use to strike hours?