D). Carcinogens are chemical factors in an organism's environment that can cause spontaneous mutation.
Motor neurons carry impulses to the effectors in the body, which are the muscles or glands that produce a change based on a stimulus sensed by the receptors. The motor neurons "innervate" muscle fibers, which essentially means that they cause them to move. The fewer the muscle fibers associated with a motor neuron, the more precise are the movements of those fibers. Having to control a small number of fibers, the neuron is able to coordinate more dexterous motions. An example of a region where there are not a lot of muscle fibers related to each neuron is in your fingers.
Because in winter 1 habitat might get too cold so the bird might have to move to a warmer habitat in winter
Introns because they are coding sequences.
Exons because they are non-coding sequences.
