Gene X always produces the same mRNA (one splice form) and the same protein in all cells in which it is expressed. The gene X mRNA and protein are present in brain, kidney, and heart cells. However, the activity associated with the protein is present only in the heart. Which of the following are possible explanations?A. A splicing factor necessary for removing all of the primary transcript's introns is present only in the heart. B. A splicing factor necessary for removing all of the primary transcript's introns is present only in the brain and kidney. C. Protein X is phosphorylated only in the heart. D. Protein X is phosphorylated only in the brain and kidney. E. A miRNA whose target is gene X mRNA is present only in the heart. F. A miRNA whose target is gene X mRNA is present only in the brain and kidney.