The cause of Osgood’s syndrome

[madhur_ahuja]

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
(B) unknown, neither do they know
(C) not known, nor do they know
(D) not known, unknown too is
(E) not known, neither do they know

[abhishek_mumbai]

B C D
who is they? pronoum error

D..
the cause is not known, unknown too is ..
Incorrect formation

IMO A

[venkpeters]

Another vote for A

[divya23]

shd be C

[hardy]

A for me. Other seems wordy.

[Target7XXHash]

Here we can rule out the options B,C and E as the word "they" does not refer to anything.
D ruled out as the sentence formation is incorrect..
IMO --A