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    In America, boys and girls often join clubs. The members of a club work together. They also learn certain skills and share good times.

    Many boys belong to Boy Scouts. Boy scouts enjoy hiking and camping out together, and they learn many things. Scouts learn safety rules, first aid, and what to do when in danger. Members have a chance to learn outdoor living. They learn to build fires and cook out in the open.

    Girl Scouts and Camp Fire Girls are clubs for girls. These two clubs give their members a chance to have fun and learn. The girls in a group take trips, have parties and go hiking and camping together. The Camp Fire Girls earn honor beads( 荣誉珠) for learning such skills as swimming and cooking. The Girl Scouts earn badges(徽章).

    Both boys and girls can be members of 4-h clubs, Children who live in either the city or the country may belong to 4-h. A project such as raising a pig or a calf can be carried on only by farm children, while city children can take part in gardening and cooking projects.

    In all these clubs the boys and girls learn to be good and useful citizens.

①外地网友邀请你去做客                          ②网友索要你的生活照片

③网友索要你的信用卡号及相关信息      ④提出网恋的要求

在实数$\sqrt{2}$，$\frac{22}{7}$，0.101001，$\sqrt{4}$中，无理数的个数是（    ）

在平整地面上有一层厚度均匀的积雪，小明用力向下踩，形成了一个下凹的脚印，如图所示，脚印下的雪由于受外力挤压可近似看成冰层，利用冰的密度，只要测量下列哪组物理量，就可以估测出积雪的密度？（　　）