物体受到三个大小分别为3N、4N、5N的共点力作用，这三个力合力的最小值是（　　）

通读短文，掌握其大意，然后在各小题所给的三个选项中，选出一个最佳答案。

    Sam is my new classmate. He is 1 American boy. He is from New York. He is thirteen.

    Sam often gets up at about 6: 30 on2. And he often 3 a cup of tea, an egg and some bread for his breakfast. Then he goes to school at 7: 30 a.m. and gets there 4 8: 00 a.m. . They have four classes in the morning and two in the 5. At 3: 50 p.m. they play games. He studies hard and studies 6. He likes English and is good at it. He teaches us 7 and we teach him Chinese. He can speak a little Chinese now, but not 8 . Mike is his good friend. They go home together(一起). He does 9 homework at home, but sometimes at school. On Saturday and Sunday, he likes 10 football games on TV. He often goes to bed at about nine.

在C+O₂CO₂反应中，____ 份质量的碳与____ 份质量的氧气反应生成____ 份质量的二氧化碳，反应中各物质的质量比为（化成最简）　____ ．