下列各式中,一定是二次根式的是( )
A:-2017
B:x3
C:x2+1
D:x+2016
All of us have been to the park ____________________ because both of you are ill at home.
我国是一个多火山、地震的国家,原因是( )
读寓言《坐井观天》说说青蛙和小鸟在争论什么?