在屈肘动作中起牵拉作用的结构是（    ）

阅读下列短文，从每小题的三个选项中选出最佳选项。

    Hello, I'm Ann. This is my family. There are five people in my family. My grandparents are old. They are teachers. My father is tall. He has a white shirt. He is a fireman. He is not afraid of fire. He can help people. He likes his job. My mother is tall and beautiful. She has long hair. She has a yellow skirt. She is a nurse. I don't have any aunts. What about you?

请选用下面的词语填空，组成一幅合乎意境的对联，正确的一项是（   ）。

上联：帘外微风斜燕影

下联：____________近____________

函数f(x)＝ln(x＋1)－$\frac{2}{x}$的零点所在的大致区间是(　　)

如图，四边形ABCD，对角线AC与BD相交于O．下列四个命题：

①若AC⊥BD，则S_{四边形ABCD}=$\frac{1}{2}$AC•BD．

②若AD∥BC，AO=CO，则四边形ABCD是平行四边形．

③四边形ABCD沿着AC折叠，能够重合，则四边形ABCD是菱形．

④若∠BAD+∠BCD=180°，四边形ABCD内一定有两对相似三角形．

其中是真命题的是____ ．（将正确的结论序号填在横线上）