Final answer:
The main difference between siRNA and miRNA is that miRNAs are encoded by the genome while siRNAs are not; both are involved in RNA interference and are processed by Dicer.
Step-by-step explanation:
The main difference between siRNA and miRNA is that miRNAs are encoded by the genome while siRNAs are not (B option). Both siRNA and miRNA are involved in the process of RNA interference (RNAi) and both are processed by an enzyme called Dicer. However, siRNAs are often exogenous or artificially introduced and are perfectly complementary to their mRNA targets, leading to mRNA cleavage when bound to RISC. In contrast, miRNAs are endogenously encoded as part of the genome and are not always perfectly complementary to the mRNA, which can lead to inhibition of translation rather than cleavage of the mRNA.