21. B. recessive
22. D. DD and dd
23. D. FF and ff
The correct answer is that the shells of clams and other mollusks dissolve when the pH becomes more acidic.
This is because the shells of clams and other mollusks are made of calcium carbonate that is dissolved in acidic conditions. The pH needs to be low to dissolve calcium carbonate into it. It takes few days to dissolve the shell completely.
the second one, "using the scientific method makes it easier for scientists to share their results"
Explanation:
all of the other answers are correct do therefore I used the process of elimination, also, scientists sharing their results is the 7th step to the scientific method, it is not the targeted end result