题目

Only when _____ in 1945 _____ to his hometown.A.was the war over; he returned  B.the war was over; had he returnedC.the war was over; he returned  D.was the war over; had he returned 答案：B把正整数排列成如图甲三角形数阵，然后擦去第偶数行中的奇数和第奇数行中的偶数，得到如图乙的三角形数阵，再把图乙中的数按从小到大的顺序排成一列，得到一个数列{an}，若an=2011，则n=    ．