平行结构 VS. 伴随

这两种形式的题目总是做错，比如313选  C,342 看成是cause 的宾语也不错啊，请教大家对这种题是怎样思考的？

313.       
Found throughout Central and South
America, sloths hang from trees by long rubbery limbs and sleep fifteen
hours a day, moving infrequently enough that two species of algae grow on
its coat and between its toes.           (D)

(A) sloths hang
from trees by long rubbery limbs and sleep fifteen hours a day, moving
infrequently enough

(B) sloths hang
from trees by long rubbery limbs, they sleep fifteen hours a day, and with such
infrequent movements

(C)
sloths use their long rubbery limbs to hang from trees,
sleep fifteen hours a day, and move so infrequently

(D) the sloth
hangs from trees by its long rubbery limbs, sleeping fifteen hours a day and
moving so infrequently（D）

342.       
If industrial pollution continues to
deplete the ozone layer, the resulting increase in ultraviolet radiation will
endanger human health, causing a rise in the incidence of skin cancer and eye
disease, and perhaps even threatening global ecological systems.    (B)

(A) and perhaps
even threatening

(B) and may even
threaten

(C)
and even a possible threat to

(D) as well as
possibly threatening（B）

(E) as well as a
possible threat to

313，不单要看平行，还要考虑逻辑。

342，如果是cause的宾语，and前的逗号就不需要了。