Cs是稳定的核电荷数最大的碱金属元素，下列关于铯及其化合物的叙述，正确的是(　　)

设$\left\{a_n\right\}$是等差数列,若$a_2=3,a_7=13$则数列$\left\{a_n\right\}$前8项和为（     ）

来自撒哈拉以南非洲的一位非洲客人解绍自己家乡情况，你认为不可能的是（　　）

It ____(snow)heavily last night．So I ____(ask)a few friends to the park this morning．The trees, the houses and the roads．____(be)all white．We ____(decide)____(make)a big snowman on the road．We ____(put)a carrot in the snowman's head．That ____(be)the snowman's nose．The two small black balls in his face were the snowman's eyes．Then we started ____(sing)and ____(dance) round the snowman．We ____(have)a great time today．