Paramecium are single cell eukaryotes that live in fresh (pure) water. They have a vacuole that contracts to expel excess water that enters the cytoplasm. Eukaryotes that live in salt water do not need these vacuoles because * 1. there is more solute inside their cytoplasm than outside it
2. there is the same amount of solute outside their cytoplasm as in it
3. salt will not enter their cytoplasm
4. water can't cross their cell membrane

C >-< sorry if its wrong im like 65% sure

Explanation: im think its C due to the fact that salt water nor any kind of water goes into cytoplasm.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds