Lternative splicing can process the same pre-mrna into different forms of mrna. which one of the following reasons best explains why this pre-mrna can't be processed into a mrna containing only (exon 1-intron a-exon 3)?
a. the spliceosome can not excise rna sequences bounded by two 5' splice sites.
b. the spliceosome can not excise rna sequences bounded by two 3' splice sites.
c. the spliceosome can not excise rna sequences bounded by an upstream 5' splice site and a downstream 3' splice site.
d. the spliceosome can not excise rna sequences bounded by an upstream 3' splice site and a downstream 5' splice site.