A、The power created by ocean waves is unstable. B、The waves are sometimes strong but sometimes weak. C、Some coastal areas are to

An often overlooked renewable energy resource is the power of ocean waves. The relentless line of waves breaking along a coastline represents a steady stream of energy.(16)The problem is that the water in waves actually goes up and down, a motion which is hard to convert into a force pushing only one way, like a flowing river or a blowing wind.
    Scientists at the University of California Berkeley designed an underwater device which they say not only solves this problem but has a dual application. "Our device has the advantage that we do not directly convert into electricity.(17)We can decide ourselves if we want to produce fresh water or electricity," Lehmann said. Their carpet-like mechanism, that rises and falls with waves, creates hydraulic pressure, pumping seawater towards the shore. The pressurized water can be used to run turbines(涡轮机), generating electricity. Or it can be pushed through special membranes(薄膜)that extract the salt to create fresh water.
    Lehmann said larger versions of the wave carpet could power small coastal communities. "So in general the available resource of wave energy is in the order of 15% of the global energy demand, which is a lot. "
    Mechanical engineer Reza Alam said to avoid possible impact on coastal ecosystems,(18)the wave carpet can be deployed in so-called " dead zones" where there is not enough oxygen for marine life to thrive.(19)"Placing a carpet on the seabed in those locations is definitely absolutely safe to the environment," he explained.
    Alam said only one-square meter of the wave carpet could supply enough power for two typical American homes, which on a larger scale means that the ocean could cheaply power up entire coastal towns.
16. What is a problem of utilizing ocean waves according to the passage?
17. What is the dual function of the underwater device designed by scientists?
18. Which area should the carpet-like mechanism be deployed?
19. What do we know about the wave carpet?