一道迷惑的语法题

the demand for airplance mechanics is expected to grow about ten percent a year in the next decade, largely becuase new federal rules calling for major modifications to older planes and because the air-lines are adding hundreds of new jets.

请问为什么正确答案是" new federal rules call for major modifications to older planes"?  原文和这个答案唯一的不同就是calling for 和 call for, 为什么要用call for的愿意呢？ 区别是？？ 谢谢指教，感激不尽！！！

明白了，因为CALLING是现在分词，而CALL FOR是谓语动词吧？

谢谢2位