Have you ever planted a few trees on Tree Planting Day? Each year, millions of people, both 1, plant a lot of trees. But this may not be the best way to make your home a 2 place.
    Last week we visited a park in Hebei. We were3 to find most of the trees had been burned. The workers told us that the trees 4soon after they were planted because they weren't watered well enough. 5 workers burned them and cleaned the place for 6 trees to be planted this year.7 a card found in one of the dead trees, a student wrote,”I hope this tree will grow up with me to 8 the backbone (栋梁) of our country.” The workers said that the survival (存活) of the trees was really more important than 9 trees were planted.
    Some people in Beijing now have a new idea that they can donate (捐赠) some money and let professional (专业人士) plant and 10 the trees.

如图所示，四边形的通电闭合线框abcd处在垂直线框平面的匀强磁场中，它受到磁场的合力（  ）