What do you do at the weekends? Some people like to1 at home. But others like to go for a walk2 play football. My friend Jack works hard in a factory on3. At the weekends he always 4 different things. On Saturday he5 his car and on6 he goes with his family to a village by car. His uncle and aunt have a farm there. It's not a7 one, but there's always8 to do on the farm. The children help with the animals and give them their9. Jack and his wife help in the field. At the end of the day, they are all10 and want to have a big meat very much.

如图，AB是⊙O的直径，弦CD⊥AB，垂足为E，若AE=2，CD=8，则⊙O的半径为（　　）

发现行星运动定律的科学家是