为了防止向员工过量支付工薪，或向不存在的员工虚假支付工薪，下列最有效的内部控制措施是()

A.资产和记录的实物控制

B.适当的凭证和记录

C.适当的授权

D.适当的职责分离

Most birds find it safe to sleep in the trees, but ____ they have eggs or young chicks, they don’t use a nest.

A．why

B．how

C．unless

D．where