下列各组物质中，前者属于单质、后者属于氧化物的是(  )

    “Leave a place better than you found it.” It is a 1 philosophy, but one that William Lindesay

    2 dear.On April 22, which was also Earth Day, he led twenty 3 to help clean up the Jiankou section of the Great Wall.

    “He who 4 is the owner,” William said in the 5 remarks of the cleanup event in 2000.“The Great Wall 6China, and is also one of the greatest 7 in the world.Everyone has a 8 to protect it.Every day should be Earth Day,” he said.

    During the past three decades, he has spent most of his time 9 and exploring the Great Wall and has developed his own 10 to protect the cultural relic.

    “It's a 11 thing that people are going out to enjoy themselves and to 12 the great history and heritage, 13 it's also quite dangerous if they don't 14 how fragile (易损坏的) the historical sites are and how fragile the 15 is,” William said.

    In 1989, his first 16Alone on the Great Wall was published, and he has published another five books about the Great Wall since then.In 2001, he 17 International Friends of the Great Wall, a(n)

    18 whose efforts are aimed at protecting this 19to the past.

    His next step is to organize exhibitions about the Great Wall as a 20 over the last 400 years.

    The exhibitions are expected to be held in Beijing, London, Paris and Washington DC.

一般地说，干旱条件下育成的作物品种，适于在干旱地区种植；潮湿条件下育成的作物品种，适于在潮湿地区种植。在这里，干旱和潮湿所起的作用是