Which one of the following statements correctly describes the splicing process undergone by most eukaryotic pre-mRNAs?

A、In a two- step reacion, the spieosome removes the exon as a larat and joins the two introns together.
B、Splicing requires conserved sequences which are the 5'-splice site, the3'-spliee site the branch-point and the polypurine tract.
C、The U1 snRNP initially binds to the 5'-plice site, U2 to the branchpoint sequence and then the tri-snRNP, U4, U5 and U6 can bind.
D、In the first step of splicing the G at the 3'-end of the intron is joined to the 2'-hydroxryl group of the A residue of the branchpoint sequence to create a lariat