31. _____ salmon find their way back to their birthplace still remains a mystery to biologists. (A) Which (B) How (C) Although (D) Since

32. Marine biologists use jellyfish, coral, and sea anemones to study bioluminescence, _____. (A) certain living organisms give off the heatless light (B) which certain living organisms give off the heatless light (C) is the heatless light given off by certain living organisms (D) the heatless light given off by living organisms