一元二次方程的根是（ ）．
A：
B：
C：
D：
黄河是中华民族的母亲河．读图，下列有关黄河流域的说法，正确的是（ ）
根据短文，现在请同学们为Mr. Cao填完整他工作日的日程安排表 Mr. Cao is a teacher in a middle school．He has a lot of things to do from Monday to Friday．On Monday and Tuesday morning he has three speaking classes．On Thursday he has two writing classes in the morning and one in the afternoon．He plays ball games after class on Monday and listens to music on Tuesday afternoon．He doesn’t have classes on Wednesday or Friday．He works on the computer on Wednesday morning．He can speak a little Japanese．So he takes Japanese classes that afternoon．On Friday morning．Mr. Cao meets old friends．In the afternoon，he has a rest at home
⑴过点C画直线CE∥OB，交OA于E；
⑵过点C画直线CF∥OA，交OB于F；
⑶过点C画线段CG⊥OA，垂足为G．
根据画图回答问题：
①线段____长就是点C到OA的距离；
②比较大小：CE____CG（填“＞”或“=”或“＜”）；
③通过度量比较∠AOD与∠ECO的关系是：∠AOD____∠ECO．
的根是（ ）．