請依下文回答第 21 題至第 25 題 　　Post-traumatic stress disorder is a malady of memory. Sufferers are often haunted by recurrent nightmares,distressing thoughts and flashbacks so intense in color, smell and sound that they feel as if they are reliving thetrauma. But what if these unbearable memories could be selectively erased? Sheena Josselyn, a professor ofphysiology and psychology, who studies how the brain encodes, stores and uses information, is intrigued bythe idea and has been investigating how to “silence” memories --make them temporarily inaccessible-- in mice.She thinks it’s possible that a variation of this technique could one day help treat post-traumatic stress disorderin humans. 　　Studies with mice have found that although their brains contain billions of neurons, only a few arenecessary to form a fearful memory. Researchers working with mice began by teaching them to fear a tone:when it sounds, they feel a mild shock to their feet (not to hurt them, just to scare them). The next time themice hear the tone, they crouch and freeze, signaling fear. The researchers discovered that they could triggerthe memory of that fear even without presenting the tone. They did this by stimulating the small group of nervecells holding that memory through a technology called optogenetics. Using the same technology, they foundthey could also suppress the fearful memory. With optogenetics, scientists insert proteins into neurons to makethem sensitive to light. Depending on the type of protein and color of light used, these cells can then beactivated or deactivated by shining pulses of the light directly into the brain. If the light activates the cells, themice freeze as if they’ve just heard the tone. If the light deactivates the cells, the memory is suppressed. Whileoptogenetics is an invasive procedure and technologically not feasible with humans, Josselyn hopes that thegeneral principles learned from these studies could eventually help scientists create new drugs for treatingmemory disorders such as post-traumatic stress disorder and Alzheimer’s. 　　But should you erase a bad memory? Absolutely not, says Josselyn. She emphasizes that this futuretechnology should not be applied to everyday bad things, and that these discoveries need to go hand in handwith a real thinking about the ethics involved in potentially manipulating memories in people. Their use wouldonly be considered in extreme cases after all other treatment options have been explored. The goal is not tosanitize life or make people super happy, but rather to make everyone a functional person, capable of momentsof joy.
21 What would be the best title of this passage?
(A) Possible Causes of Memory Loss.
(B) Post-traumatic Stress Disorder.
(C) Studies of Trauma in Humans.
(D) Suppressing Fearful Memories.