Which one of the following about enzymes is true?

(Refer: DM Vasudevan, Textbook of Biochemistry, 7th edition, pg no: 59 - 60) The Michaelis constant (km) Km provides a measure of the affinity of the enzyme for its substrate. It is equal to substrate concentration at which reaction velocity is equal to ½ Vmax. Km = substrate concentration when reaction velocity is % Vmax. Km is constant for any given enzyme. Km is independent of substrate or enzyme concentration. The higher is KM, the higher is the concentration of the substrate required to reach ½ Vmax. That is: at a lower affinity, a higher concentration of substrate is required to reach ½  Vmax. ↓ Km = ↑ affinity.