The DNA of Gene X in Humans Has Been Isolated

The DNA of gene X in humans has been isolated, cloned, and sequenced and is 8000 base pairs in length. From cells that were actively transcribing gene X, a cDNA library was made. In this case, gene X was found to produce from a mRNA that was 4200 bases in length. The most probable explanation of these results is that

A) the DNA encoded introns that were excised during mRNA processing.
B) a mutation resulted in a large loss of this particular gene.
C) reverse transcription of the mRNA isolate was not complete.
D) there was no mRNA in the cells of the mRNA used to make the cDNA library.
E) none of the above