OG11th-69关于名词的单复数判断？

69. A report by the American Academy for the Advancement of Science has concluded that much of the currently
            uncontrolled dioxins to which North Americans are
            exposed comes from the incineration of wastes.

(A)         much of the currently uncontrolled dioxins to which North Americans are exposed comes

(B)         much of the currently uncontrolled dioxins that North Americans are exposed to come

(C)        much of the dioxins that are currently uncontrolled and that North Americans are exposed to comes

(D)        many of the dioxins that are currently uncontrolled and North Americans are exposed to come

(E)         many of the currently uncontrolled dioxins to which North Americans are exposed come

解释说。。。 many must be used for a countable quantity such as people or dioxins。。。

dioxins为什么做复数名词用啊，不理解，有人讲讲吗，先谢了！

查一下词典，dioxin可以有plural form....

所以用dioxins...