Sentence Improvement - Verbal and Reasoning

Directions to SolveIn questions given below, a part of the sentence isitalicised and underlined. Below are given alternatives to the italicised part which may improve the sentence. Choose the correct alternative. In case no improvement is needed, option 'D' is the answer.
Q1:

Why the dinosaurs died out is not known.

A it is not known

B the reason is not known

C that is not known

D No improvement

ANS:D - No improvement

No answer description is available.