A professor of environmental health and his students began studying the idea of rooftop gardening for cities. “I think vertical farming is an idea that can work in a big way,” said Despommier.

Why would we want to build skyscrapers filled with vegetables when we’ve been farming on the ground for 10,000 years? Because as the world’s population grows—from 7 billion now to as much as 9 billion by 2050—we could run out of productive soil and water. Most of the population growth will occur in cities that can’t easily feed themselves. And the fact is that modern agriculture as well as everything associated with it – chemical fertilizers and transportation full of carbon—is a significant contributor to climate change, and suddenly vertical farming doesn’t seem so magic in the sky.

“Vertical farming could allow food to be grown locally and sustainably,” says Glen Kertz, CEO of Valcent. The result saves space — vital in urban areas — and allows farmers to irrigate and fertilize with far less waste.

At Valcent’s El Paso lab, crops grow in rows on conveyor belts. Moving them gives the plants the precise amount of light and nutrients needed, letting them grow 15 times as much vegetable as on a normal farm with 5% of the water that traditional agriculture does.

Despommier plans to build a 30-story, city-block-size vertical farm that would have transparent walls to maximize sunlight and would produce enough food for 50, 000 people. But it would cost hundreds of millions to build a full-scale skyscraper farm. That’s the main drawback: construction and energy costs would probably make vertically raised food more costly than traditional crops.

静脉注射时，先用胶皮管扎紧上臂，在扎紧处一端静脉隆起，使静脉隆起的结构是（　　）

A．动脉瓣

B．静脉瓣

C．房室瓣

D．毛细血管瓣