Yellow jackets number among the 900 or so species of the world's social wasps, wasps that live in a hilghly cooperative and organized society consisting almost entirely of females-the queen and her sterile female workers.

请问这里的number among。。。wasps是什么意思呢？为什么不选A， wasps living in a highly cooperative and organized society where they consist almost entirely of ?

谢谢。