[单项选择]If you go down to the woods today, you may meet high-tech trees-genetically modified to speed their growth or improve the quality of their wood. Genetically-engineered food crops have become increasingly common, albeit controversial, over the past ten years. But genetic engineering of trees has lagged behind.
Part of the reason is technical. Understanding, and then altering, the genes of a big pine tree are more complex than creating a better tomato. While tomatoes sprout happily, and rapidly, in the laboratory, growing a whole tree from a single, genetically altered cell in a test tube is a tricky process that takes years, not months. Moreover, little is known about tree genes. Some trees, such as pine trees, have a lot of DNA-roughly ten times as much as human. And, where- as the Human Genome Project is more than halfway through its task of isolating and sequencing the estimated 100,000 genes in human cells, similar efforts to analyze tree genes are still just saplings (幼苗)
A. began much later
B. has developed more slowly
C. is less useful
D. is less controversial

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