题目

阅读下面短文，从短文后备题所给的A、B、C、D四个选项中，选出可填入空白处的最佳选项。 （淮安）    Wishing to encourage her young son's progress on the piano, a mother took her boy to a pianist's concert on a summer holiday evening. After they found their ____16____, the mother saw two friends in the hall and walked to ___17___ them. It was the boy's first time to come to the hall. He thought it was a good ___18___ for him to explore the wonders of the concert hall. He ___19___ and walked around.  He walked ___20___ a door marked NO ADMITTANCE(1．若已知氢原子核外电子在离核为r0处受到的库仑力大小为F，则它在离核为4r0处受到的库仑力大小为（　　）A．$\frac{1}{16}F$B．$\frac{1}{4}F$C．16FD．4F