By the first century A.D, historians say, the Chinese had some knowledge of the winds and

currents of the Pacific. When weather prevented the Chinese from navigating by the sky their mariners(水手) used magnetized(磁化) needles to guide them. Because the sailors had no level surface on their rocking ship, they floated the needle in a shallow bowl of water, and the compass that guided explorers to day was born.

Between 1450 and 1433, Emperor Chu Ti's Treasure Fleet of the Dragon Throne ruled the entire south Pacific and the Indian Ocean, collecting gifts and tributes(贡品) for the Emperor throughout a territory that ranges from Korea and Japan to Eastern coast of Africa. The Admiral Cheng Ho began the first of seven voyages in 1405. The purpose of the first voyage was to hurt the former emperor who had been dethroned and escaped. The admiral's fleet, which was made up of 62 large junks and 255 smaller ships, recaptured the deposed emperor and also defeated a large pirate fleet that had been terrorizing the islands near Sumatra. Over the next 28 years, Admiral Cheng Ho increased China's trading territory to including 37 countries from the Vietnam coast to Persian Gulf, the Red Sea, and eastern Africa. Cheng He's death around 1435 signaled the end of the Treasure Fleet and disappearing of China's economic domination of the South Pacific.

It is implied but not stated that ancient navigation used to sail ______.

A．with their knowledge of the winds

B．by the sky

C．by the compass

D．by the currents of the Pacific