. I think that bivalves have evolved to have smaller head regions because they have also evolved to have bigger shells or valves than cephalopod
Answer: i would say the answer is D
Explanation: srry if i got it wrong
IT WOULD BE 15 CAUSE 10+5 IS 15 AND ITS THE SAME AMOUNT GOING IN AS OUT AND 15 IS GOING OUT SO 15 IS GOING IN.