We had to wait half an hour we had already booked(预定) a table.
A:since
B:although
C:until
D:before
次氯酸钠(NaClO)是某种家用消毒液的主要成分,其中氯元素的化合价为( )
夏朝建立于约公元前2070年,这一年属于公元前( )