The underlined word "leg" in Paragraph I probably means ______.

A．a dam will be built across the river
B．a dam will store water for dry seasons
C．in wet seasons a dam will prevent the flooding of the land
D．houses and fields will be covered with water

A．the problems caused by river water
B．the necessity to irrigate the field
C．the water shortage in some countries
D．the canals carrying water to fields