有些射性元素要经历一系列的衰变才能稳定，关于X、Y、Z三个原子核的衰变方和如下：

X→ ${}_{82}^{206}$ Pb+8 ${}_2^4$ He+6 ${}_{-1}^0$ e    

Y→ ${}_{82}^{207}$ Pb+7 ${}_2^4$ He+4 ${}_{-1}^0$ e 

Z→ ${}_{82}^{208}$ Pb+6 ${}_2^4$ He+4 ${}_{-1}^0$ e

根据以上方程可以推断在X、Y和Z中，下列正确的是（   ）

    Most American families like to have a vacation in summer. Summer is a good season for vacation. It is often hot in July and August.

    Children do not go to school in those two months. Some people like to stay at home, read books, or watch TV. Many families take their lunch to eat at a place with many trees or a nice lake. Some people have enough time and money to travel to other countries like France, Japan and Australia. They usually fly to these countries. Many families travel by car or train to see interesting places in their own countries. Their favorite city is New York.

    Not everyone likes to go to busy cities. Some families travel to mountains or beautiful valleys（山谷）.

在思想品德课堂上，老师要求根据下列图片概括成就主题，你认为最合适的是（    ）

下列标点符号使用不正确的一项是  （    ）