Nancy and Sheila are in the same class. They are friends. Nancy wants to invite Sheila to dinner on Sunday. But Sheila doesn't know the way. Now Nancy is telling her.

"It's easy. You can take No. 4 bus. After you get off at Red Road. cross it and take the first turning on the left. Walk along the road for about five minutes. and then you'll see a big tree. Go straight on for about a hundred meters. and you will see a big red house. This is not my home. Go past the big house about four hundred meters. and you'll see a small yellow house beside a little tree. Then you can open the door with your foot."0

    "With my foot?" asks Sheila. "Why?"

    "Well. you won't come to my house empty-handed(空手地)， will you?"  answers Nancy.

2014年《财富》世界500强企业中中国的上榜企业创纪录地达到100家，位列世界第二位，中石化成为世界排名第三的企业。这（   ）

①意味着我国已跻身发达国家行列

②显示了社会主义制度强大的生命力

③说明我国已经跨越社会主义初级阶段

④得益于我国坚持以经济建设为中心

某同学利用托盘天平、玻璃杯、水、细棉线、记号笔，测出一块小石块的密度，他主要实验步骤如下：

（1）用调好的天平测出石块的质量m₁，并记录在表格内；

（2）在玻璃杯中倒入适量水，用细棉线系住石块缓慢地 ____在水中，标记此时水面的位置，用天平测出玻璃杯、水和石块的总质量为m₂，并记录在表格内；

（3）取出石块，再向玻璃杯中加水，使水面回到标记的位置，然后再用天平测出玻璃杯和水的质量m₃，并记录在表格内；

（4）石块的密度的表达式为ρ_石= ____