Read the following passage and then mark the letter A, B, C, or D on your answer sheet to indicate the correct answer to each of the questions.

Commonly defined as unwanted sound, noise is a widely recognized form of pollution. It is not easy to measure because the experienced discomfort by different individuals is highly subjective and, therefore, variable. Exposure to lower levels of noise may be slightly irritating, whereas exposure to higher levels may actually cause hearing loss, a nonfatal health problem. Particularly, the noise produced as a byproduct in congested urban areas causes physical and psychological harm, and detracts from the quality of life for those who are exposed to 'it'.

Unlike the eyes covered by the eyelids against strong light, the ears have no lids, and are, therefore, always open and vulnerable; noise penetrates without protection. Deafening noise causes effects that the hearer cannot control and to which the body hardly gets accustomed. Loud noises instinctively signal danger to any organism with a hearing mechanism, including human beings. In response, heartbeat and respiration accelerate, blood vessels constrict, the skin pales, and muscles tense. In fact, there is a general increase in functioning brought about by the flow of adrenaline which is released to respond the fear. Since noise is unavoidable in complex and industrial societies, we are constantly responding in the same way that we would react against danger more than annoyance. It is time to solve this serious problem.

According to the passage, what is noise?