Hi Could you solve my doubts!!!

Here is the rate chart for "please explain correct answer" service (20% more for people who are taking GMAT in two weeks ) :
1) $5 for an answer
2) $20 for a correct answer with explanation
3) Dumb looks and occational "I guess (A)" answers are free !

6. The cause of Osgood’s syndrome is unknown, nor is it known why the disease appears to be increasing in the United States.
(A) unknown, nor is it known - CORRECT
(B) unknown, neither do they know (Who are they ?)
(C) not known, nor do they know (Who are they ?)
(D) not known, unknown too is (This creates a run-on sentence)
(E) not known, neither do they know (who are they?)

18. There has been a change in how men think of land, a change from thinking it simply private property to that now of a limited national resource, with an inherent public interest transcending the individual owner’s prerogatives.

(A) There has been a change in how men think of land, a change from thinking it simply private property to that now of (a change from etc cannot requalify (act as appositive of) land)
(B) There have been changes in how men think of land, a change from that of simple private property to now that of (a change from etc cannot requalify land)
(C) There has been a change in how men think of land, a change from once considering it simply private property to now considering it (a change from etc cannot requalify land)
(D) Once men used to think of land as simply private property, but they consider it now (It doesn't have a clear antecedent)
(E) Once thought of simply as private property(correct modifier of land), land is now considered (CORRECT)


Unlike the law protecting endangered animals, the Fish and Wildlife Service’s listing of endangered plants does not prohibit collectors to take listed plants from the wild if in doing so they comply with state laws.
This question tests the idiom "prohibit from" so only C & D survives.
(A) to take listed plants from the wild if in doing so they comply with state laws
(B) from taking listed plants from the wild if they are so doing in compliance with state laws (so doing in compliance etc are grammatically incorrect)
(C) from taking listed plants from the wild if in doing so they comply with state laws (CORRECT)
(D) who take listed plants from the wild should they be in compliance with state laws
(E) that take listed plants from the wild so long as they are doing so by being in compliance with state laws