A:①
B:②
C:③
①长城 ②金字塔 ③都江堰——青城山 ④秦始皇陵及兵马佣 ⑤峨眉山——乐山大佛
You'd better put your heart into study. You see, the ________ you are, the ______ progress (进步) you'll make.
(I)求{an}的通项公式;
(II)设{an}的前n项和Sn=155,求n的值.