Insurance is the sharing of risks. Nearly everyone is exposed to risk of some sort. The house owner, for example, knows that his property can be damaged by fire; the ship- owner knows that his vessel may be lost at sea; the breadwinner knows that he may die at an early age and leave his family poorer. On the other hand, not every house is damaged by fire, not every vessel lost at sea. If these persons each put a small sum into a pool (集体储备金), there will be enough to meet the needs of the few who do suffer loss. In other words, the losses of the few are met from the contributions of the many. This is the basis of insurance. Those who administer the pool of contributions are insurers.
The legal basis of all insurance is the policy. This is a printed form of contract on stout paper of the best quality. It states that in return for the regular payment by the insured of a named sum of money, called the premium which is usually paid every year, the insurer will pay a sum of money or compensation for loss, if the risk or insured against actually happens.
The premium for an insurance naturally depends upon how likely the risk is to happen, as suggested by past experience. If companies fix their premiums too high, there will be more competition in their branch of insurance and they may lose business. On the other hand if they make the premium too low, they will lose money and may even have to drop out of business. So the ordinary forces of supply and demand keep premiums at a level satisfactory to both the insurer and the insured.By "the pool of contributions" the writer means（）

A．the cost of administering insurance
B．the a mount of each premium
C．money paid by the insurer
D．money paid by the insured