Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the option that best fits each of the numbered blanks from 18 to 22.

Stress
In prehistoric times, the physical changes in response to stress were an essential adaptation for meeting natural threats. The stress response, even in the modern world, (18) _____ during critical events such as sports activities, important meetings, or situations of actual danger. If stress becomes persistent and low-level, however, the brain, heart, lungs, vessels and muscles become chronically over- or under-activated. (19) _____. Acute stress can also be harmful in certain situations.
Studies suggest that (20) _____ is associated with depression or anxiety. In one study, two-thirds of subjects who experienced a stressful situation had nearly six times the risk of developing depression within that month. Some evidence suggests that repeated release of stress hormones disrupts normal levels of serotonin, the nerve chemical
(21) _____ . Certainly, on a more obvious level, stress diminishes the quality of life by reducing feelings of pleasure and accomplishment, and relationships are often threatened.
The tensions of unresolved stress frequently cause insomnia, generally keeping the stressed person awake or causing awakening in the middle of the night or early morning. In fact, evidence suggests that stress hormones can increase during sleep in anticipation of a specific waking time. (22) _____ . Relaxation therapy has been found to reduce stress levels and consequently improve the quality of sleep.
(Adapted from Cambridge Vocabulary for IELTS)