A、It is hard to clean up. B、It can’t be recycled. C、It can produce a greenhouse gas. D、It can’t turn into electricity. C新闻中提到垃圾放

In Disney’s cartoon Wall-E, the robot is left behind on a deserted Earth to clean up the waste mankind left behind. If the latest statistics are anything to go by, we’re in danger of turning that fictional future into a reality.
    A decade ago, city residents generated 680 million tonnes of solid waste a year. [3]Now this is 1.3 billion tonnes, and forecast to rise to 2.2 billion tonnes by 2025.
But could technology, which helped create much of this waste, also help deal with it?
    [4]Much of the world’s waste goes to landfill sites, which only add to the pollution problem because they produce methane—a greenhouse gas and significant contributor to climate change.
    But technology is helping to extract the gas and turn it into electricity. For example, US-based Ener-Core has built installations at landfill sites in California and the Netherlands that can produce between 250 kilowatts and 1 megawatt of electricity.
3. What is the problem mentioned in the news report?
4. What problem does the waste going to landfill sites cause?