Dams are barriers that prevent the flow of water. they are created to provide flood control, recreational areas, water for irrigation, and hydroelectric power. although dams serve many important purposes, they can also have a negative impact on the environment by a. serving as a place for people to have picnics. b. increasing fresh water sources for humans. c. providing new wildlife habitats. d. preventing fish from moving along a river.

As Dams act as Obstruction to the Natural Flow of the River, they Prevent fish from moving along the River (i.e.) either from Sea to River or From River to Sea.

So : Option D is the Answer

the abdomen (less formally called the belly, stomach, tummy or midriff) constitutes the part of the body between the thorax (chest) and pelvis, in humans and in other vertebrates. the region occupied by the abdomen is termed the abdominal cavity. the stomach secretes acid and enzymes that digest food. ridges of muscle tissue called rugae line the stomach. the stomach muscles contract periodically, churning food to enhance digestion. the pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine.