如图，有质量分别为m₁、m₂的两个物体通过轻弹簧连接，在力F的作用下一起沿水平方向做匀加速直线运动（m₁在光滑地面上，m₂在空中）．已知力F与水平方向的夹角为θ．则m₁的加速度大小为（   ）

    Mary is a little girl. She is only four. She is not at school. She doesn't know how to read or write. But her sister Joan is a schoolgirl. She is ten years old. She knows how to read and write. One day, Joan sees her little sister in the room. She's at the table. There's a pencil in her hand. She's writing. “What are you writing, Mary?” She asks. “I'm writing a letter to my friend Rose. ” “But you don't know how to write. ”says her sister. “Well,” says Mary, "it doesn't matter. Rose doesn't know how to read. "