The children (clean) the blackboard they had used for their Mathematics exercise. ( Fill in the blanks using simple past tense)

The children cleaned the blackboard they had used for their Mathematics exercise.

one of the central themes of edward abbey's work has been his staunch opposition to the glen canyon dam on the colorado river that created lake powell, destroying stunning scenery and wildlife habitat, endangering many rare species of fish and animals, and also irreparably destroying many important archaeological sites which provided information about the early inhabitants of the united states.

i don't understand : /