题目内容：
Researchers have met with difficulties when trying to make algae produce hydrogen efficiently. Which one of the following is one such difficulty? _______
A．A It is not possible to remove sulfate from the environmen B．B It is not possible to work in an airfree environment to produce hydroge
C．C It is not easy to make sugars instead of hydroge
D．D It is too slow for algae to produce hydrogen when the sulfate is remove
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