A cDNA library is synthesized from mature polyadenylated mRNA and therefore exclusively contains the coding sequences (exons without introns) of active genes expressed in a given tissue. When an organism's genome sequence is unknown, isolating a functional protein-coding gene directly from genomic DNA is hindered by vast non-coding intergenic regions and introns. Furthermore, gene-specific primers cannot be designed without prior sequence data. Thus, screening a cDNA library (e.g., using heterologous probes, antibody screening of expression libraries, or differential display) represents the classical and standard approach to isolate protein-coding genes.

Both statements are correct:
Statement (A): Unlike replicative DNA polymerases (which possess intrinsic $3' \to 5'$ exonuclease proofreading activity), core RNA polymerases lack dedicated intrinsic $3' \to 5'$ exonuclease proofreading nuclease domains.
Statement (B): Because of the absence of rigorous exonuclease proofreading and post-synthetic mismatch repair systems for transcripts, RNA synthesis exhibits an error rate of approximately 1 in $10^4$ to $10^5$ nucleotides, which is substantially higher than the error rate of DNA replication (approximately 1 in $10^9$ to $10^{10}$ with proofreading and MMR).

The Cauliflower Mosaic Virus (CaMV) 35S promoter is the most widely used promoter in plant genetic engineering. It is a strong, constitutive promoter that drives high-level continuous gene expression across virtually all cell types, tissues, and developmental stages in both dicotyledonous and monocotyledonous plants, independent of environmental stimuli.