A sheet of metal was shaken to ______ the sound of thunder.

A．trying to add a little humor
B．trying to illustrate his earlier point
C．simply joking
D．being religious

A．those viruses carried in the blood of animals
B．some viruses which are at present harmless to man
C．the viruses that reproduce in man without making him ill
D．the still-not-yet-discovered viruses in the jungle