请教一道语法题pp test2-13

Cotton is grown throughout the world, and __year about 50million bales, weighing nearly 500 pounds apiece, are produced.

each

a

by a

in whcih

答案是A，我认为是D，为什么一个句子里有两个动词is 和are呢？

and前是一句，and后又是一句，“weighing nearly 500 pounds apiece”是插入语，不用理它。

哎哟，真是弱啊！

谢谢两位大虾！！