Read the following passage and mark the letter A, B, C or D to indicate the correct answer to each of the questions from 21 to 25.

Old newspapers, magazines and other unwanted papers are collected by the rag-and-bone man. The recovered paper is then sorted according to the grade or type of paper before being sent to the recycling centre. There, the sorted papers are gathered and bundled into tight bales before they are transported to a paper manufacturing factory where they will be recycled into new paper. Only paper free of contaminants such as food, plastic, metal and other rubbish can be successfully recycled; otherwise, impurities and bacteria might be introduced into the recycling process and thereby affect the quality of the paper produced.

At the paper mill, recovered paper is chopped up and mixed with water to form pulp slurry, which is then subjected to a series of washing where soap-like chemicals called surfactants remove ink from the paper. After the deinking process, the pulp slurry is sprayed onto a huge flat wire screen that is rapidly moving through the paper machine. Water drains out and fibres bond together into a web of paper which is pressed between rollers to squeeze out more water and smoothen the surface. Heated rollers dry the paper, which is then slit into smaller rolls or sheets and finally removed from the paper machine.

Papermaking fibres can be typically recycled five to seven times before they become too short and are rejected by the paper machine. Recycling paper certainly benefits the environment because it requires at least 50 per cent less energy and up to 75 per cent less water than making paper from virgin fibres. Also, it reduces the demand for virgin fibre in the world.

So, have you done your part to save Mother Earth?

What does "recovered paper" in paragraph 1 refer to?

The building has a smoke detector ________ any fires can be detected immediately.