20. The answer is 1 (Precambrian)
21. The answer is 4 (Pennsylvanian)
22. The answer is 2 (Bony Fishes, Dinosaurs, and Mammals)
23. The answer is 2 (Reptiles)
The answer is the septic tank or A
The answer is hydrogen bonds
The basic building block of a protein is called an amino acid. There are 20amino acids in the proteins you eat and in the proteins within your body, and they link together to form large protein molecules.
