Read the following passage and mark the letter A, B, C, or D to indicate the correct answer to each of the questions.

A theme park in France has started a new and environmentally-friendly way of dealing with litter. It has trained six "intelligent" rooks to pick up rubbish. Rooks are members of the crow family. The birds will work at the Puy du Fou theme park in west France. They have been trained to pick up cigarette ends, paper and other things thrown on the ground. The birds pick up the litter and put it into a special trash can. This delivers bird food as a reward. The more litter the birds put in the trash can, the more food they get.

The head of the theme park explained why it is using the crows. He said there were several reasons. He said: "The goal is not just to clean up ... but also to demonstrate that nature itself can teach US to take care of the environment." Crows are very sociable and intelligent birds. They may be as smart as a seven­-year-old child. The theme park head said: "In an affectionate, supportive atmosphere, they like to communicate with humans and establish a relationship through play." The crows are quick workers. They can fill a bucket with rubbish in nearly 45 minutes.

(breakingnewsenglish.om > environment-2018)

What is the theme park keeping free of litter?