题目内容：
请根据短文内容，回答题。Putting Plants to Work
Using the power of the sun is nothing new. People have had solar-powered calculators and buildings with solar panels (太阳能电池板) for decades. But plants are the real experts. They've been using sunlight as an energy source for billions of years.
Cells in the green leaves of plants work like tiny factories to convert sunlight, carbon dioxide(二氧化碳), and water into sugars and starches( 淀粉 ), stored energy that the plants can use. This conversion process is called photosynthesis (光合作用 ) . Unfortunately, unless you're a plant, it's difficult and expensive to convert sunlight into storable energy. That's why scientists are taking a closer look at exactly how plants do it.
Some scientists are trying to get plants, or biological cells that act like plants, to work as very small photosynthesis power stations. For example, Maria Ghirardi of the National Renewable Energy Laboratory in Golden, Colo, is working with green algae (水藻). She's trying to trick them into producing hydrogen instead of sugars when they perform photosynthesis. Once the researchers can get the algae working efficiently, the hydrogen that they produce could be used to power fuel cells in cars or to generate electricity.
The algae are grown in narrow-necked glass bottles to produce hydrogen in the lab. During photosynthesis, plants normally make sugars or starches. "But under certain conditions, a lot of algae are able to use the sunlight energy not to store starch, but to make hydrogen," Ghirardi says.
For example, algae will produce hydrogen in an airfree environment. It's the oxygen in the air that prevents algae from making hydrogen most of the time.
Working in an airfree environment, however, is difficult. It's not a practical way to produce cheap energy. But Ghirardi and her colleagues have discovered that by removing a chemical called sulfate ( 硫酸盐) from the environment that the algae grow in, they will make hydrogen instead of sugars, even when air is present.
Unfortunately, removing the sulfate also makes the algae's cells work very slowly, and not much hydrogen is produced. Still, the researchers see this as a first step in their goal to produce hydrogen efficiently from algae. With more work, they may be able to speed the cells' activity and produce larger quantities of hydrogen.
The researchers hope that algae will one day be an easy-to-use fuel source. The organisms are cheap to get and to feed, Ghirardi says, and they can grow almost anywhere: "You can grow them in a reactor, in a pond. You can grow them in the ocean. There's a lot of flexibility in how you can use these organisms."
How do plants relate to solar energy? A．They are the real experts in producing it.
B．They have been a source of it.
C．They have been used to produce it.
D．They have been using it for billions of years.
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