It is necessary that he (come)（）back without delay.

A．Vases with a light blue background.
B．Paintings of flowers.
C．Paintings of birds.
D．Vases with a dark blue background.

A．At 6:00 a.m.
B．At 6:00 p.m.
C．At 6:30 a.m.
D．At 6:30 p.m.