While I was waiting to enter university, I saw an advertisement in a newspaper –a teaching job wanted at a school.  The school was about ten miles from where I lived. I was in great 1of money and wanted to do something useful, so I2.
    Three days later, a letter arrived.  It asked me to meet the principal of the school at Croydon. It proved to be a(n)3journey -a train to Croydon station, a ten-minute bus ride and then a walk of4a quarter of a mile.
    A man who was short and round seemed to be the principal and he opened the door for me. "The school," he said, "has a group of twenty-four boys between seven and thirteen years old." According to the man, I should have to teach all the subjects5art, which the man himself taught. I should have to6the class into three groups and teach them at three different levels, and I was not glad at the7of teaching Maths -a subject which I wasn't good at. It was8that I had to teach them on Saturday afternoon because most of my friends would be9themselves at that time.
    Before I had time to ask about my10,the man got up to his feet. "Now," he said,“You'd better meet my wife.  She is the one who really runs this school.”

关于如图所示的实验装置，下列说法正确的是（   ）

已知椭圆的参数方程 （t为参数)点M、N在椭圆上，对应参数分别为 ， ，则直线MN的斜率为____。