"Class Nouns" such as Clothing, Food, Furniture, Apparel, Hardware, Equipment, Software etc. are always SINGULAR and must therefore take a SINGULAR verb.

You CANNOT end a modifier with a preposition.

Examples:
"dioxins that north americans are exposed to" -- INCORRECT.
"dioxins to which north americans are exposed" -- CORRECT

Gmat sc 1000 example question

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

The OA is E.

Two points:

first, dioxins are countable (the fact that it's pluralized means that it's a countable noun). The sentence is referring to different dioxins, not the overall level of dioxin (which would be uncountable).

Second, "to which North Americans are exposed" not only uses the right idiom, it also avoids the issue of dangling participles (although the GMAT doesn't usually test on that issue, it's technically incorrect to end a sentence or clause with a preposition).

Phrase/Clause as SUBJECT:

The subject of a sentence is sometimes an entire phrase/clause. However, they are ALWAYS SINGULAR and require only SINGULAR VERBS.

Examples:
Having good experience in a field does bring in great opportunities.
Whatever they want to do is fine with me.