if a sentence contains a COMPOUND NOUN (X and Y), you CANNOT use a pronoun to refer to just X or just Y, unless you use "the former" or "the latter" to make the distinction explicit.
in (d), it's technically incorrect to use "they" because there is no place in which both systems are mentioned at once (e.g., with "and").

D，they无指代

in (b) and (d), the pronoun "its" is completely stranded; there's no antecedent for it anywhere.
takeaway:
if a sentence contains a COMPOUND NOUN (X and Y), you CANNOT use a pronoun to refer to just X or just Y, unless you use "the former" or "the latter" to make the distinction explicit.

* see #109 in the yellow og11. in that problem, all four of the incorrect answers try to use the singular pronoun "his" to refer to 1/2 of a compound noun. that's forbidden.
DO NOT post additional details of that problem here.

* this rule still holds even if X and Y take separate pronouns.
for instance:
joe and jane both have prestigious jobs, although her salary is much higher
would still be disallowed under the strict rules generally endorsed by the gmat, even though "her" can only refer to jane (since joe is male).
in this case, you'd just have to substitute "jane's" for "her".
that's the way it goes.

in (d), it's technically incorrect to use "they" because there is no place in which both systems are mentioned at once (e.g., with "and").

takeaway:
if a sentence contains a COMPOUND NOUN (X and Y), you CANNOT use a pronoun to refer to just X or just Y, unless you use "the former" or "the latter" to make the distinction explicit.

takeaway:
if a sentence contains a COMPOUND NOUN (X and Y), you CANNOT use a pronoun to refer to just X or just Y, unless you use "the former" or "the latter" to make the distinction explicit.

and because of its 中的its没有指代，在BD选项中

单复数

不选b 是因为both 。。。 and。。。，b曲解了原意

"because all of its neural pathways converge"中的its暗示之前一句because从句主语应该为The human nervous system (the former of The human nervous system and a telephone system)

(D) 副词superficially位置改变，修饰对象有歧义；they没有指代对象