The key answer of exercise 1

from top to bottom

• Photo 1: this illustrates the risk of terrorists taking explosives in cans or tubes on to a plane – fluids and pastes are currently prohibited from flights
• Photo 2: this illustrates the need to wear visible clothing in the street. These school kids have been given hi-vis jackets, presumably so that they won’t be hit by motorists, or perhaps so that the teachers can make sure they don’t lose any kids.
• Photo 3: this illustrates the risk involved in putting up scaffolding in a high rise building
• Photo 4: this is a joke. It is making fun of the fact that signs are often put up to warn us of unimportant things or unlikely risks – here, the sign is warning us about itself.