The diagram shown here is part of an RNA transcript containing four open reading frames (M, H, K, and L) and three introns (1, 2, 3). The sites labeled d1-d3 are nucleotides immediately before each intron on the 5' side and those labeled a1-a3 are nucleotides immediately after each intron on the 3' side. Splicing out an intron involves a spliceosome facilitating the cleavage of the DNA at the d and a sites at either end of the intron. How many processed transcripts are possible in which two d sites and two a sites are cleaved?