阅读下面短文, 按照句子结构的语法性和上下文连贯的要求, 在空格处填入一个适当的词或使用括号中词语的正确形式填空。

    Sara and her father planned ____ trip to a mountain near their home. The path they chose____ (begin) at the foot of the mountain and led all the way ____ the top. Sara packed lunches earlier that morning. ____ (she) father filled their backpacks with other things and they  ____ (be) off!

    As they started up the path, Sara’s father pointed out many different____ (plant) that grew on the path. He showed Sara the ____ (different) between the____ (leaf) of trees.  Sara was glad ____ her father knew so much! She always learnt something whenever they went on a trip. This time, she was sure that she ____ learn more.

真空中有两个静止的点电荷，它们之间的作用力为F，若它们的带电量都增大为原来的2倍，距离减小为原来的 $\frac{1}{2}$ ，它们之间的相互作用力变为____F．