如图某三角形展览馆由25个正三角形展室组成，每两个相邻展室(指有公共边的小三角形)都有门相通，若某参观者不愿返回已参观过的展室(通过每个房间至少一次)，那么他至多能参观多少个展室（）
A．23
B．22
C．21
D．20
参考答案：C
解析：
如图对展室作黑白相间染色，得10个白室、15个黑室，按要求不返回参观过的展室，因此，参观时必定是从黑室到白室或从白室到黑室(不会出现从黑到黑，或从白到白)，由于白室只有10个，为使参观的展室最多，只能从黑室开始，顺次经过所有的白室，最终到达黑室，所以至多能参观到21个展室，选C。
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| After my 21 second graders finished reciting the text, they settled back in their seats.But Duane remained_1_.Duane was a bright and lovable student, _2_his mother, a single parent, had many problems such as drinking._3_that he might have a bad night, I walked over to him to see what was wrong.As he looked up, I could see the_4_in his dark eyes. "Mrs Brown, aren't you going to open my present?" he asked_5_."I put it on your desk. Getting back his gift from my desk, he handed it to me.I noticed my gift_6_to be a matchbox. Duane told me that this was really a jewelry box_7_a matchbox.As I opened it, the_8_of two beer caps surprised me.Duane_9_me that they were two silver earrings.He had noticed that I 10 wore earrings and wanted me to have some pretty ones. I was 11 by Duane's creativity and thoughtfulness. 12 birth, one of my ears was slightly deformed (畸形的). Fearing that wearing earrings might 13 to the ear, I avoided wearing them. But how could I 14 to wear these precious earrings given by this 15 child? As I placed the earrings on my ears, my 16 clapped, and Duane stood proudly beside me. Since then, the matchbox remained on my desk.It 17 me of Duane's act of kindness and of the lessons he taught me.Although his 18 at home was bad, Duane continued to see the good in life.Although poor, he still wanted to 19 .Whenever I see Duane's gift on my desk, I feel encouraged. If I am having 20 reaching a student, I'll try to be like Duane and give that student a piece of my heart  . | ||||
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