Questions  70-73 are based on the following passage.

    As its name suggests, hydroculture typically involves growing plants in nutrient-rich water rather than in soil. Hydroponics is the most widely-known method of hydroculture. Some believe it has significant potential to reduce the impact of the food crisis by allowing farmers to produce large quantities of food in a sustainable and cost-effective way. In a typical hydroponics system, crops are grown indoors in controlled environments. The plants grown in containers filled with water and the nutrients that they need to thrive. These nutrients vary depending on the crop being grown but typically include nitrogen, potassium, phosphorus, iron, zinc, and copper, among other trace elements. A light source provides the energy the sun would usually deliver.

    In so-called static systems, the nutrient solution is changed or replaced either according to a schedule or when the plants require it. In continuous-flow systems, the nutrient solution flows constantly past the roots of the plants. Although it may seem like a technologically sophisticated form of agriculture, scientists have been experimenting with hydroculture since at least 1627. Early experiments showed that plants grow best when raised in water rich in various nutrients rather than ini distilled water. By the early decades of the twentieth century, the concept of hydroponics was becoming more widely known and some commercial applications of the technology were even in place. These days, NASA, the US government agency in charge of space missions, is even researching hydroponics.

    Hydroponics offers a number of benefits over conventional agriculture. It happens indoors, and so does not require large areas of land. In fact, hydroponics farms can be located in industrial warehouses in or near large cities. This gives them another advantage over traditional farms since they can be located much closer to the markets for their produce. reducing distribution and transportation costs for hydroponics farmers. The indoor location allows farmers to create controlled environments. This means not only does the weather not affect crops, but farmers can grow crops that would not normally thrive in a particular region, such as tropical vegetables in Canada. But hydroponics farms are not without their drawbacks. For one thing, some important food crops do not grow well hydroponically. Most herbs and vegetables thrive, but important sources of nutrition like carrots, 236 potatoes, and corn are either not suitable or hard to grow. And so much space would be required to raise staple crops like rice and wheat cost-effectively that their hydroponic cultivation is not practical. Further, most foods that grow well hydroponically are relatively low in protein, a vital nutrient for human health.

    A variation on hydroponics that solves some of these issues is aquaponics. (1) A basic aquaponics system is a hydroponics farms with a second layers. On top are plants growing hydroponically in nutrient-rich water. (2) A below that is a tank of fish being raised as part of a fish farms. In traditional aquaculture, farmers must spend time and money cleaning the water in which the fish live, if they do not, the health of the fish will suffer as waste from the fish concentrates in water. (3) The clever part of aquaponics is that instead of having to filter and discard the fish waste, farmers can use it as the source of nutrients of the plants. (4) By closing the system in this way, aquaponics farms can produce the same kinds of herbs, fruits, and vegetables as hydroponics farms, as well as high-quality protein from farm-raised fish, too. (5)

Why does the author mention "tropical vegetables" in paragraph 3?