Mouse meta-globin mRNA problem
There are always exceptions but you can consider some general rules.

A is 1 i.e the sequence that is perfectly complementary (2 is complementary but direction is parallel — cannot base-pair), then B would be 1 (siRNA). Reasons (some are just based on general rules of siRNA design):


siRNAs have to be perfectly complementary
The size is also perfect for a siRNA (21nt)
U/A (2-3 residues) at the 5' of the siRNA
U at 10th position
GC content 43% (ideal 30-50%)


See here for guidelines for siRNA design.