The most important use of drifting (漂流) bottles is to find ocean currents. When the position and direction of currents are known, ships can use the forward movement of a current or stay away from currents that would carry them off their course. Benjamin Franklin was one of the first to use bottles in the study of currents. He wondered why British mall ships needed a week or two longer than U.S. ships needed in order to cross the Atlantic Ocean. Franklin thought the Gulf Stream (墨西哥湾流) might explain this difference.
Franklin talked with captains of U.S. ships. He found that they knew each turn of the Gulf Stream. They used the current in every possible way. From his talks with the captains. Franklin made his first map of the Gulf Stream. Then he checked his map by using sealed (密封的) bottles. The map that he finally made is still used, with only a few changes, today. 
【小题1】 Why are drifting bottles used?
A．To determine the position of a ship
B．To carry message across the ocean
C．To predict the direction of a ship.
D．To find the direction of a current
【小题2】What led Franklin to talk with U.S. captains?
A．U.S. ships were longer than British ones.
B．British ships could sail the Atlantic faster than U.S. ones.
C．U.S captains knew more about maps.
D．U.S. ships could sail the Atlantic faster than U.S. ones.
【小题3】What did Franklin make after his talks with U.S. captains?
A．A map of the Atlantic Ocean.
B．A map of the Gulf Stream..
C．A map of ocean currents.
D．A map of his first voyage.
【小题4】What did Franklin do in order to make an exact map?
A. He compared his own map with other maps
B. He talked with many U.S. captains.
C. Both B and C.
D. He used drifting bottles to check his map.