Which of the following is mentioned in the passage to imply that the use of tidal turbines hasn’t been popular for a very long t

Listen to part of a lecture in an environmental engineering class, then answer the following questions.
Professor: Recently, we’ve been talking about finding renewable sources of energy. Well, we’ve done solar and wind energy, so today we could talk about ways to harvest energy from the sea. Specifically, let’s focus on the tidal currents that exist in the ocean.
    Tidal energy (also called "tidal power") is a form of hydropower which converts the energy of tides into electricity or other useful forms of power. One way to get energy from tidal currents is to use tidal turbines. When the currents pass through the turbines, they will turn the turbines, which then will generate electricity. A tidal turbine looks a lot more like a windmill under the water, and it actually works in pretty much a similar way. The idea isn’t that innovative since it’s been discussed for quite some time now. Recently we have been able to create materials that can withstand the corrosive damage by salt water during extended submersion, and it makes the use of the turbines more prevalent. So the total availability of tidal power may be much higher than previously assumed, and economic and environmental costs may be brought down to affordable levels.
Student: Hold on. But tidal currents move so slowly. How could they produce electricity? I mean, doesn’t wind move a lot faster than water?
Professor: Good question. Well, simply put, water is over hundred times denser than wind. Even with a slower current, you still have enough force to produce a large amount of power. Actually, a 10km/h tidal current can generate enough electricity to match a wind current of 300km/h.
Student: Wow, I had no idea. And, um, now that I think about it, I guess wind isn’t that reliable either, is it? Sometimes it just stops blowing.
Professor: Very good point, and that is exactly why tidal energy is a better source. Tidal currents are predictable, day to day, all year round. They never change. And another advantage of using them is that tidal turbines can even change their direction underneath the water, so they can turn to their face into or out of the direction of the current. But these aren’t the first attempts to using energy from the ocean’s tides; there have been some earlier methods that I want to talk about now.
    First of all, there are tidal dams, for instance. A tidal dam is built on an inlet, and when the tide comes in, the dam will fill up. On the other hand, when the tide goes back out, the water is forced over the turbines, turning them. There are a few of them still in use right now, but they haven’t been scaled for commercial use. Tidal dams are also built in tidal basins. These areas have fragile ecosystems that could be damaged by tidal dams. The dams would affect the sedimentation and various other components in the ecosystem.
    Now, another method is called "tidal fences." Um, they stick down about 3 meters into the water, and they have turning rotors on them that are connected to two bodies of land, um, such as a mainland and an island, or just two islands. As the tide passes through the fence, it turns the rotors and produce electricity. However, tidal fences also have a downside. They can block off the entire space between the two land masses, which could have negative impact on the shipping industry that might use the corridor, But they are cheaper than tidal dams, and they don’t affect the ecosystem. Well, I suppose they could affect the movement of very large marine mammals, but it’s still less damage done than what a tidal dam would do. In order to reduce fish fatality, fences could be engineered so that the spaces between caisson wall and the rotor are large enough to allow fish to pass through.
    Okay, so now let’s get back to tidal turbines. Now I think tidal turbines are really the best way to get energy out of the tides. You see, the turbines are placed 20 meters below sea level, so tidal turbines are out of sight. They won’t ruin the aesthetic beauty of the environment. The blades have round edges and turn very slowly, so the negative side effect to the local wildlife is relatively minor. At the moment the problem is that the installation and maintenance of tidal turbines is very expensive, but this is a temporary problem as technology develops over time. But the financing of such endeavors is still unwarranted today. I’m certain that in the future it will be looked into more seriously as the world’s energy demands become more serious.