What is the significance of centralized cash reserves with the central bank? What is selective credit control?

The central bank is the custodian of the cash reserves of the commercial bank. Every commercial bank in the country has to keep a minimum percentage of cash with the central bank by law. These centralized cash reserves are of extreme importance in the following manner:

a. It helps the central bank of the country to control the money supply.

b. They can be used efficiently during the financial crisis.

c. It becomes an additional source of income to the commercial bank if it faces a short-term shortage of money.

d. They form the basis of a large credit structure which is better than the same amount being scattered among the individual banks

e. It is a source of great to the banking system/