单项选择题
A.pick up speed where the channel widens B.run slower in the center of the channel C.hug the inside of a bend D.cause the greatest depth of water to be along the outside of a bend
A.at times of new and full Moon B.when the Earth，Moon，and Sun are lying approximately on the same line C.when the Moon is between first quarter and full and between third quarter and new D.when the Moon is between new and first quarter and between full and third quarter
A.in an area of high density the water's surface is lower than in an area of low density B.surface water flows from an area of high density to one of low density C.the lesser the density gradient the freer the water is to move D.it is the density differences that cause the currents to stay in the troughs