下列对氨羧配位剂的叙述中，不正确的是（）。

A.氨羧配位剂是一类有机通用型配位剂

B.氨羧配位剂是一类含有胺基和羧基的有机通用型配位剂

C.氨羧配位剂中只含有胺基和羧基，不含其它基团

D.最常用的氨羧配位剂是EDTA

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.