A:①②③
B:②③④
C:①③④
D:①②③④
等差数列an的前n项和为Sn,若a3+a7+a11=12,则S13等于( )
曾纪泽说:“办洋务的难处,在于外国人的不讲道理,中国人的不明时势,只有徐图自强,才能扭转形势.”洋务派“徐图自强”的最早措施是( )
-- It's snowing. I really want to run outside and make a snowman.
-- Why not have a snow ball fight instead? It's more exciting.
发电的基本原理之一是电磁感应,发现电磁感应现象的科学家是()