His family           the zoo last week.

    During holidays，Kitty usually goes to see the exhibits in many museums，such as the history museum，the art museum，the museum of science and technology and the museum of natural history．There are thousands of wonderful things in museum.There are paintings，sculptures(雕塑)，costumes(服饰)，swords(剑)，ancient animal bones and plants，stuffed animals(动物标本)and many others．It is always a good review of history to visit those museums．It is also a great way to learn more things by myself．

自行车、摩托车、小汽车是我国居民出行的主要交通工具。在二十多年前，城市居民普遍用的还是自行车，而今摩托车已经普及，小汽车也已经进入城市一般家庭。这从一个侧面表明(     )

公差d≠0的等差数列{a_n}的前n项和为S_n，若向量$\overrightarrow{O{A}_1}$=$\left(a_i,\frac{S_i}{i}\right)\left(i=1,2,3\right)$,t为实数，若$\overrightarrow{A_1{A}_2}$=t$\overrightarrow{A_2{A}_3}$，则t=（　　）