1) WPC means woman police constable2) A shop-lifter is a person who steals something from shops3) They don't arrest the boys by the river because they are not doing anything wrong (they are just waiting for some friends)4) A mugger is a person who attacks and robs people in a public place.5) Yes, the mugger spoke to the old man because the old man could define his accent (Scottish)6) She arrested just one person7) She knew it was the wrong man because he spoke in an American accent, while the mugger has a strong Scottish accent