The iris in the human eye contracts and expands, controlling the amount of light that reaches the retina. what types of muscle control this? skeletal and voluntary smooth and cardiac skeletal and smooth smooth and involuntary

smooth and involuntary

The iris in the human eye contracts and expands, controlling the amount of light that reaches the retina. It is because of the activity of the smooth muscles, these muscles are present in the retina of the eye. These muscles alters the shape of the lens by contraction and expansion. These actions of  muscles are involuntary, which means these occur without the conscious choice of the organism. Other examples include breathing, digestion and heart beating.

the element that provides strength to the exoskeletons of clams and oysters is phosphorus.

this is so because calcium carbonate is the main component in the shells. calcium carbonate is strengthened by phosphorus.

explanation:

stimulus is a change in the environment that causes an organism to react. the response is the reaction to the stimulus. some examples of stimuli for zebras are drought, to many predators, and bad feeding grounds such as dead grass or plants. zebras respond to stimuli just like all other living organisms