Which of the Following Mechanisms May Be Responsible for the Occurrance

Which of the following mechanisms may be responsible for the occurrance of exon shuffling?

A) double crossover could promote the insertion of an exon into another gene.
B) nonhomologous recombination.
C) transposable elements may promote the movement of exons.
D) both nonhomologous recombination and transposable elements.
E) none of these are mechanisms responsible for exon shuffling.