单选题 Scientists have been struggling to find out the reason behind blushing （脸红）。Why would humans evolve（进化） a 21__________ that puts us at a social disadvantage by 22__________ us to reveal that we have cheated or lied?Charles Darwin pointed out that 23__________ all people of all races blush，animals do not. When it came to explaining the reason, he was 24 __________ a loss. That has not stopped others from trying.One 25 __________ is that blushing started out as a way to show we obey authoritative members of the group. Later, as our social interactions became 26__________ complex, it became involved with higher, self-conscious 27__________ such as guilt, shame and embarrassment. This would seem to put individuals at a disadvantage，but blushing might actually make a person more 28 __________ or socially desirable.29__________ that women blush more than men，one university researcher suggests that blushing might have evolved as a way for women to 30__________ their honesty to men so as to win their support in raising children. Some zoologists also think blushing could have emerged as a way to foster trust. " If you were to go hunting 31__________ a stone-faced partner，you could never 32__________ what he wants. “he says.Once blushing became 33 __________ with embarrassment, anyone who did not blush might have been at a disadvantage because we are 34 __________ likely to trust someone who appears never to feel  ____请作答此空______ about anything.