According to the writer, insurance is __________请根据以下内容回答问题Insurance (保险)is the sharing of risks. Nearly everyone isexposed to risk of some sort. The house owner, for example, knows that hisproperty can be damaged by fire; the ship-owner knows that his vessel may belost at sea; the breadwinner knows that he may die at any early age and leavehis family poorer. On the other hand, not every house is damaged by fire norevery vessel lost at sea. If these persons, each put a small sum into a pool,there will be enough to meet the needs of the few who do suffer loss. In otherwords, the losses of the few are met from the contributions of the many. Thisis the basis of insurance. Those who pay the contribution are known as insuredand those who administer(管理)the pool of contributionsas insurers.The legal basis of all insurance is thepolicy. 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 sumof money, called the pre- mium(保险费), which is usually paid every year, the insurer will pay a sum ofmoney or compensation for loss, if the risk or event insured against actuallyhappens. The premium for an insurance naturally depends upon how likely therisk is to happen, as suggested by past experience, ff companies fix theirpremiums too high, there will be more to make the premium too low, they willlose money and may even have to drop out of busi- ness. So the ordinary forcesof supply and demand keep premiums at a level satisfactory to both the insurerand the insured.According to the writer, insurance is possible because__________

A. useful but too risky
B. useful and necessary
C. costly but worth buying
D. cheap but not necessary