听力原文：LECTURER: Mum. What you have so far is ideal for the home or office but what about commercial applications? TONY: Well, we're going to demonstrate the nutrient film technique which is popular with some commercial growers, particularly for the cultivation of crops such as runner beans and tomatoes. Producers would really like to see some innovation in the cultivation of potatoes and yams too but obviously this technique is only suitable for those crops which grow above ground. LECTURER: What about peas? TONY: Well, we found peas were awfully tricky to grow using this method although we're still unsure as to the reason. LECTURER: Where's the exhibit? I don't see it anywhere. ANNIE: Well, that's because we haven't finished it yet because we're going to have to procure some mature plants first. We didn't think far enough ahead to have started them off earlier. And when we do get them, we'll have to handle them carefully. TONY: Yes, because when our model is finished, you'll see how the plants are held in position by a plastic tube which almost encloses them completely and is quite loosely fastened around the stems. LECTURER: Yes, and the feeding or watering system? ANNIE: Well, it's a bit different—on a large scale like this, you need to have the food solution trickle down through the tubes... TONY: Yes, but the solution must also be rich in oxygen... LECTURER: And what—it just bathes the roots? TONY: That's one way of doing it. LECTURER: What's the other way? TONY: You can have the solution moistening a substratum of rock wool at the bottom of the container. LECTURER: Rock wool? TONY: It's the same as mineral wool—you know a lightweight, fibrous material—the kind of thing used for insulation. Or, you can use a layer of paper fiber. LECTURER: Oh, yes...something that has the capacity to absorb the solution, right? ANNIE: Well, you do need to experiment a little. For example, we tried coconut fiber... TONY: But it just didn't have the properties we were looking for... LECTURER: I see. TONY: In our display you'll only see the basics—the kind of thing that can be done at home in the back yard, but commercial enterprises do need a lot more equipment and the media used in substrata are constantly changing as new developments are made. The cost of upgrading is ongoing. ANNIE: And there's always the potential of outright failure when changing systems from one you know and understand to an innovative one. LECTURER: Of course. There's no room for guesswork in business—you have to try to get everything right first time. Which TWO kinds of produce are especially suited to the nutrient film technique?