The ducks are swimming in the river.为什么用介词in,而不用on

The ducks are swimming in the river.为什么用介词in,而不用on 是The ducks are swimming on the river 还是in the river?是There a there are some ducks in the river are ducks any in river there the(?) Those ducks are (swimming) in the river.对括号部分提问 in,swimming,the,are,pond,ducks,the.连词成句 The boys are swimming in the river （ ）There are many ducks ____ the river.A.in B.on C.over （There are some ducks ）in the river.划线提问 _____ fish are swimming in the river. They (swim;are swimming)in the river Some ducks are swimming in the pond对some ducks进行划线部分提问