Given are two simple sentences, decipher which of the following is correctly combined complex sentence by using a noun clause?
Separate: She has failed. The reason is her illness.

A. The reason of her failure is that she is ill.

B. She was ill and thus failed.

C. She failed because of her illness.

D. The reason behind her failure is her illness.

Please do not use chat terms. Example: avoid using "grt" instead of "great".