Students these days often have a lot of worries. Sometimes they have problems with their schoolwork, and sometimes with their friends.      Some people think the worst thing is to do nothing. Laura Mills, a teenager from Washington, agrees. “Problems and worries are normal in life,” says Laura. “But I think talking to someone helps a lot. If we don't talk to someone, we'll certainly feel worse.”
    Laura once lost her purse, and worried for days. She was afraid to tell her parents shout it. She even walked three miles to school each day because she didn't have any money. She just kept thinking. “If I tell my parents, they'll be angry!” But in the end, she talked to her parents and they were really understanding. Her dad said he sometimes made careless mistakes himself. They got her a new purse and asked her to be more careful. “I will always remember to share my problems in the future!” Laura says.
       Robert Hunt advises students about common problems. He feels the same way as Laura. “It is best not to avoid our problems. We should always try to solve (解决) them.” He thinks that you can first find someone to talk to. This person doesn't need to be an expert (专家) like himself. Students often forget that their parents have more experience than them, and are always there to help them.
    In English, we say that sharing a problem is like cutting it in half. So you're halfway to solving a problem if you talk to someone about it!

下列四种调查：①了解一批炮弹的命中精度；②调查全国中学生的上网情况；③审查某文章中的错别字；④考查某种农作物的长势．
其中适合做抽样调查的个数有

已知A、B两种物质反应生成C：A+B→C．用7gA和2gB或4gA和5gB充分反应都可以得到6gC．现取A、B固体混合物共9g，充分反应后得到7.5gC．此混合物中A和B的质量分别是　____ ．