题目

设集合S＝{1,2,3,4,5,6,7,8,9}，集合A＝{a1，a2，a3}，A⊆S，a1，a2，a3满足a1<a2<a3且a3－a2≤6，那么满足条件的集合A的个数为( ) A．84  B．83  C．78  D．76 答案：B 由题意知当a3＝9时，a2＝3，a2＝4，a2＝5，a2＝6，a2＝7，a2＝8，其对应的a1可以有2＋3＋4＋5＋6＋7＝27种，当a3＝8时a2＝2,3,4,5,6,7，对应的a1有1＋2＋3＋4＋5＋6＝21种，当a3＝7时，a2＝2,3,4,5,6，对应的a1有1＋2＋3＋4＋5＝15种，依此类推得满足条件的集合A的个数为27＋21＋15＋10＋6＋3＋1＝83种_____ is expected, according to the online survey, is that prices of the houses won’t rise any more．    A． What            B． It            C． As            D． Which