【GMAT】Since 1992, in an attempt to build up the Atlantic salmon population in each of the seven rivers in which salmon still spawn, state officials in Maine have stocked them with fry raised in hatcheries from eggs produced by wild fish found in that particular river. 

老师这个空格只划了一个单词但是我感觉好像都没什么问题，为什么只能选each?
Since 1992, in an attempt to build up the Atlantic salmon population in each of the seven rivers in which salmon still spawn, state officials in Maine have stocked them with fry raised in hatcheries from eggs produced by wild fish found in that particular river. 
（OG19 V 240）
A them
B the river
C the rivers
D each river
E that river