ok, first things first.
the CORRECT answer to this problem is, unfortunately, the missing answer choice (e):
(E) many of the currently uncontrolled dioxins to which North Americans are exposed come
--
also:
you cannot end a modifier with a preposition.so:
dioxins that north americans are exposed to --> INCORRECT
dioxins to which north americans are exposed --> correct.
etc.
for an official example, see #16 in the first edition purple Verbal OG Supplement (= #20 in the second edition blue one).
now for your questions:
the problem also appears in OG. for D, OG say we have to repeat that in the clause that North Americans are exposed to.
1# if context force us to parrallel the two ralative clauses--dioxins that only trees can absorb and that North Americans are exposed to, is it ok? what about dioxins that ozones can interact with and that North Americans are exposed to,if it is not OK, how to change?
all of these examples are inherently wrong, because they contain modifiers that end with prepositions (
contra what i wrote above). therefore, i'll just use new ones.
basically, it's ok to omit "that"
if the sentence is still legitimately parallel without it.
for instance:
food that is nutritious for cats but that is poisonous for dogsfood that is nutritious for cats but is poisonous for dogsfood that is nutritious for cats but poisonous for dogsall correct.
hand signs that natives use and that appear in local rituals --> correct.
these are both [i]that-clauses that modify "hand signs", so the parallel structure works.
BUT
hand signs that natives use and appear in local rituals --> INCORRECT.
the second of these doesn't work because the right-hand component of the parallel structure is
appear in local rituals (i.e., active verb whose subject is "hand signs"). there is no verb in the left-hand side that is parallel to this, so this makes no sense.
2# regardless the prepositions, if the clause is dioxins that are uncontrolled and North Americans face, is it still wrong? in another word, should it be dioxins that are uncontrolled and that North Americans face?
yes.
see the "hand signs" example -- same thing, pretty much exactly.
i am sorry for asking several seemly unrelated questions; acctually, i just want to go one step from the test problem and thus to learn more actively.
nah, these are good questions.