题目

阅读理解 At your next meeting, wait for a pause in conversation and try to measure how long it lasts. Among English speakers, chances are that it will be a second or two at most. But while this pattern may be universal, our awareness of silence differs dramatically across cultures. What one culture considers a confusing or awkward pause may be seen by others as valuable moment of reflection and a sign of respect for what the last speaker has said. Research in Dutch (荷兰语) and also in English found that when silence in conversation stretches to four seconds, people start to feel uneasy已知f是集合M={a，b，c，d}到集合N={0，1，2}的映射，f（a）+f（b）+f（c）+f（d）=4，则不同的映射有________．