Which paragraph best summarizes the passage? The most primitive time-measuring device was a pole planted upright in a sunny place that would cast a shadow on the ground. The position of that shadow marked time. Another ancient invention for measuring time was the water-clock. These primitive techniques were used for hundreds of years before modern clocks were invented. It is impossible to establish by whom, when, or where clocks were invented, but it is fairly clear that in AD 1000, a monk named Gerbert made a clock for the German city of Magdeburg. Clocks may have been made before this, but there is no evidence to establish this fact. Unlike modern clocks, the first clocks invented were large and stationary. Later, the bulky, stationary clocks were replaced by more portable ones. Portable clocks appeared around the beginning of the 14th century, though the inventor and the exact time or place of invention of these portable clocks is unknown. The invention of portable clocks marks an era in the development of the clock. However, the introduction of the pendulum was the greatest event in clock development. This invention has been credited to Huygens, a Dutch philosopher, who not only discovered the pendulum but also put it to use in 1657. The modern watch is a developed type of the clock. The earlier clocks had originally only one hand, which indicated the hour. Minute and second hands were added later. Within recent years, clocks operated with electricity have been invented. With the beginning of clock and watch manufacture by machinery, the cost has been so reduced that practically anyone may own an accurate time-piece. The United States is one of the foremost countries of the world in the manufacture and sale of clocks as well as watches.

A - The most primitive time-measuring device was a pole planted upright in a sunny place that would cast a shadow on the ground. The position of that shadow marked time. Another ancient invention for measuring time was the water-clock. These primitive techniques were used for hundreds of years before modern clocks were invented.

B - It is impossible to establish by whom, when, or where clocks were invented, but it is fairly clear that in AD 1000, a monk named Gerbert made a clock for the German city of Magdeburg. Clocks may have been made before this, but there is no evidence to establish this fact. Unlike modern clocks, the first clocks invented were large and stationary.

C - Later, the bulky, stationary clocks were replaced by more portable ones. Portable clocks appeared around the beginning of the 14th century, though the inventor and the exact time or place of invention of these portable clocks is unknown. The invention of portable clocks marks an era in the development of the clock. However, the introduction of the pendulum was the greatest event in clock development. This invention has been credited to Huygens, a Dutch philosopher, who not only discovered the pendulum but also put it to use in 1657.

D - The modern watch is a developed type of the clock. The earlier clocks had originally only one hand, which indicated the hour. Minute and second hands were added later. Within recent years, clocks operated with electricity have been invented. With the beginning of clock and watch manufacture by machinery, the cost has been so reduced that practically anyone may own an accurate time-piece. The United States is one of the foremost countries of the world in the manufacture and sale of clocks as well as watches.