Read the following essay. Choose the best way to complete each of the following sentences. Clinical depression Many people tend to misuse the term depression to refer to the normal ups and downs of daily life. In reality, depression is a serious illness. A clinically depressed person is in a constant state of sadness because of three main factors: genetics ( 遗传学 ), substance abuse ( 药物滥用 ), or environment. Perhaps the most common cause of depression is genetics. People who are born with low levels of serotonin ( 血清素 ) and dopamine ( 多巴胺 ) cannot experience pleasure in the same way that balanced people can. People with low levels of these substances do not experience happiness from typically happy s. Another cause of depression is substance abuse. When drugs enter the bloodstream, they alter the brain’s normal chemical balance. Consequently, people who use these chemical substances may experience short-term or long-term depression due to the chemical changes in their brains. As a case in point, an alcoholic ( 酗酒者 ) can develop depression because of the constant changing of the levels of dopamine in his or her brain. Finally, environmental factors can trigger clinical depression. Failed relationships, such as a divorce or a falling out between family members, can leave a person in a state of depression in which the person is unable to handle himself or herself. Traumatic ( 造成精神创伤的 ) s, such as the death of a family member or the witnessing of a murder, are environmental factors that can send a person into depression. Regardless of its cause, depression is a serious illness that afflicts millions of people throughout the world. And it needs to be taken seriously.