截至2014年底，我国私营企业和个体工商户超过6400万户，民间投资占全部投资的65%，非公有制企业提供了80%以上的就业岗位。这表明非公有制经济（    ）

在﹣2，π，2x，x+1，$\frac{xy}{2}$中，代数式有（　　）

    It was Saturday. Mike stayed at home alone(单独地). His parents went to visit his grandparents in another city. He did his homework and read books. After lunch, he watched TV and played basketball on the playground with Jack. He felt very tired. In the evening, his parents came back and brought him some presents. They were a schoolbag and a basketball. Mike felt very excited!

在生态学研究中，下列方法与研究目的不相符的是