绿色植物的光合作用可用式子表示为：二氧化碳+X 有机物+氧气．式子中“X”表示的物质是（  ）

如图，在冷空气层中喷洒干冰（固态二氧化碳）是“人工增雨”的一种常用方法，请用学过的物理知识解释“人工增雨”的原理．

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    The White House is an important historical building in the USA. It lies1the country's capital. Many people expect2see the palace with their own eyes.

    We3six months getting tickets to visit the White House. I was 4with excitement while we were waiting in the White House South Lawns. The first thing5interested me was the colorful flowers and green grass.

    Everything inside and outside was6kept in the white House. We went7many security checks(安检)8entering the building. In the east hallway9the first floor, I saw the pictures of past presidents on the walls. The10had over 2700 books. The Vermeil Room(金银屋) and China Room（瓷器屋） showed different11

    We walked upstairs to the State Floor. The East Room, the12in the White House, was the most wonderful in its size and furniture. This is the room where the president usually13the reporters. The Green, Blue and Red Rooms on the same floor got their names because of the 1414of the furniture and walls. These rooms are15having parties for state guests.