题目

When people think of working animals，what often comes to mind are dogs that watch sheep，horses that work on farms and animals that perform in movies.But there are lots of other jobs animals have had over the years.Dogs have greater sense of smell than humans.This made dogs the traditional hunting partners，making their owners able to follow foxes and other animals.Police departments have taken advantage of this skill to help find missing people and escaped criminals.Recently，dogs have been trained to find drugs and explosives (爆炸物).The honey－guide lives in the southeast African 7．下列有机反应属于取代反应的是（　　）A．B．CH2=CH2$\stackrel{催化剂}{→}$ C．CH3COOH+CH3CH2OH$?_{△}^{浓硫酸}$CH3COOCH2CH3+H2OD．C2H5OH+3O2$\stackrel{点燃}{→}$2CO2+3H2O