A.mark   B. checked  C. population   D. destroyed  E.chemicals

In Tanzania, an African country, a company called APOPO trains African pouched rats(非洲有袋鼠）to find landmines(地雷）left over from the wars. The rats are trained to smell out some of the_小题1:__  in them. More importantly, these rats are too light to set off the landmines.
Generally speaking, after 8 to 12 months of training, the rats can find landmines so that they can be safely destroyed. Rats and trainers search one small area at a time. To be sure that every inch of the ground is _小题2:_, APOPO workers use a harness-and-rope system that guides the rats back and forth in rows. The trainer stands safely on the edge(边缘) of the area.
When a rat smells landmines, it will dig the dirt in that spot(地点). The trainers __小题3:_ each spot on a map. Later, people set off the landmines safely.
In the African country of Mozambique, nearly half of the ___小题4:__ are younger than 15 years old. A lot of kids are in danger as they play and work. With the help of the rats and trainers, there will be no threat any more.