下列句中成语使用不正确的一项是（    ）。

地表常流性河道频率指的是以一直线截取某一地区，求取被直线切割的河道数与该直线长度之比。下图表示我国年降水量、年径流曲线与河道频率的南北地带变化。读图，回答下列问题。

观察“长江三角洲简图”，回答问题。

完型填空
   When I was walking down the street the other day, I happened to1a small brown purse2 on the road. I3it up and opened it to see if I could4 the owner's name. There was5 inside it6 some changes (零钱) and an old photo--a picture of a woman and a young girl of about twelve years old who looked7 the woman's daughter. I put the photo back and8 the purse to police station, 9 I handed it to an officer. Before I left, the officer took my name and address10the owner might want to write and thank me.