17. Please put the following steps of Cap synthesis in the correct order. 　 1: N7 of the capping guanine is methylated. 　 2: The terminal phosphate is removed from the pre-mRNA. 　 3: A capping GMP is added to the pre-mRNA. 　 4: The 2'-O-methyl group of the penultimate nucleotide is methylated. (A) 1, 2, 3, 4 (B) 1, 4, 3, 2 (C) 2, 4, 1, 3 (D) 2, 3, 1, 4 (E) 4, 3, 2, 1