Why do they feel upset?

(Understanding)

Shihab and Kaniz see people bathing, washing clothes, and throwing kitchen rubbish in the river.

They see some polythene bags, plastic bottles and some other things floating on the river.

Fish cannot swim easily in dirty water. They get caught into polythene bags. So their lives are in danger.

People throw different types of rubbish into the river. Thus, the water is polluted.