Mary is a student. She enjoys ____(run). Every morning, she starts to run near her home____ (介词) six o'clock. She also ____ (like) ball games. After school, she ____ (join) her classmates to play basketball. She ____ (eat) well too. She has three ____ (meal) at school. Her favourite food is ____(dumpling). But the dining hall is small with only ten ____ (table). The students are always ____(wait) there. Now it's time for dinner. Mary is ____ (walk) to the dining hall.

已知双曲线的标准方程为$\frac{x^2}{9}-\frac{y^2}{16}=1$，F为其右焦点，A₁，A₂是实轴的两端点，设P为双曲线上不同于A₁，A₂的任意一点，直线A₁P，A₂P与直线x=a分别交于两点M，N，若$\overrightarrow{FM}$$·$$\overrightarrow{FN}$，则a的值为（　　）