43. Conditioning is a way of learning by association, _____, by putting together different ideas. (A) there is (B) that is (C) so as (D) that were

40. Scientists say feral pigeons are a valuable way of probing contaminants in environment, ___ they are exposed to the same air, water, food and other factors as humans, and don’t venture far from home. (A) although (B) hence (C) because of (D) since