鼻窦黏液囊肿的发病机制为（）

A.鼻窦的自然开口长期阻塞

B.窦壁黏膜下腺体轻度炎症或变态反应

C.乳牙根管感染

D.龋齿使牙髓慢性感染坏死

E.窦壁黏膜下变态反应

Insurance is the sharing of (1) . Nearly everyone is exposed (2) risk of some sort. The house owner, for example, knows that his (3) can be damaged by fire; the ship owner knows that his vessel may be lost at sea; the breadwinner knows that he may die by (4) and (5) his family in poverty. On the other hand, not every house is damaged by fire or every vessel lost at sea. If these persons each put a (6) stun of money into a pool, there will be enough to (7) the needs of the few who do suffer (8) . In other words the losses of the few are met from the contributions of the (9) . This is the basis of (10) . Those who pay the contributions are known as (11) and those who administer the pool of the contributions as insurer.
The (12) for an insurance naturally depends on how the risk is to happen as suggested (13) past experience. If the companies fix their premiums too (14) , there will be more competition in their branch of insurance and they may lose (15) . On the other hand, if they make the premiums too low, they will not have (16) and may even have to drop out (17) business. So the ordinary forces of supply and (18) keep premiums at a proper (19) to both insurers and those who (20) insurance.