已知某项工程由甲、乙两队合做12天可以完成，乙队单独完成这项工程所需时间是甲队单独完成这项工程所需时间的2倍少10天．甲、乙两队单独完成这项工程分别需要多少天？设甲队单独完成需x天，根据题意列出的方程正确的是（　　）

根据材料内容，选择正确答案。

    Mr. Brown is an old man. Every morning he goes for a walk in the park. And he comes home at twelve thirty for his lunch in the middle of the day.

But today a car stopped at his house at twelve 0'clock. Two policemen helped him get out. One of them said to Mr. Brown's daughter, "He can't find his way home in the park and ask us to take him home in the car."

    The daughter thanked the policemen and they leave. Then she asked her father, "Dad, you go to the park every day. But today you can't find the way. What's wrong?" The old man smiled like a child and said, "I can find my way home. But I don't want to walk home."

无土栽培是现代农业的一项新技术，其营养液是多种化合物的混合溶液，含有植物生长所需的营养元素．某种植物进行无土栽培时，需含氮、钾等元素的中性或弱酸性营养液，下列能用来配制营养液的一组化合物是（  ）